Update some grid docs

After a couple reports (fixes #23754 (closed) and closes #24076 (closed)), I'm clarifying some things in the How it works section of the docs. I'm doing my best to be very specific and explain grid concepts and terms where we use them. However, not everything can be explained here—working with Bootstrap does require some experience with HTML, CSS, and building responsive web projects.

#24076 (closed) is only getting closed here because it's factually inaccurate. I don't disagree with the tone of it or think it's inappropriate. There's language like this all over the docs.

docs/4.0/layout/grid.md

@@ -8,9 +8,12 @@ toc: true
 
 ## How it works
 
-Bootstrap's grid system uses a series of containers, rows, and columns to layout and align content. It's built with flexbox and is fully responsive. Below is an example and an in-depth look at how the grid comes together.
+Bootstrap's grid system uses a series of containers, rows, and columns to layout and align content. It's built with [flexbox](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Using_CSS_flexible_boxes) and is fully responsive. Below is an example and an in-depth look at how the grid comes together.
 
-<div class="bd-example bd-example-row">
+**New to or unfamiliar with flexbox?** [Read this CSS Tricks flexbox guide](https://css-tricks.com/snippets/css/a-guide-to-flexbox/#flexbox-background) for background, terminology, guidelines, and code snippets.
+
+<div class="bd-example-row">
+{% example html %}
 <div class="container">
   <div class="row">
     <div class="col-sm">
@@ -24,27 +27,26 @@ Bootstrap's grid system uses a series of containers, rows, and columns to layout
     </div>
   </div>
 </div>
+{% endexample %}
 </div>
 
 The above example creates three equal-width columns on small, medium, large, and extra large devices using our predefined grid classes. Those columns are centered in the page with the parent `.container`.
 
 Breaking it down, here's how it works:
 
-- Containers provide a means to center your site's contents. Use `.container` for fixed width or `.container-fluid` for full width.
-- Rows are horizontal groups of columns that ensure your columns are lined up properly. We use the negative margin method on `.row` to ensure all your content is aligned properly down the left side.
-- Content should be placed within columns, and only columns may be immediate children of rows.
-- Thanks to flexbox, grid columns without a set width will automatically layout with equal widths. For example, four instances of `.col-sm` will each automatically be 25% wide for small breakpoints.
-- Column classes indicate the number of columns you'd like to use out of the possible 12 per row. So, if you want three equal-width columns, you can use `.col-sm-4`.
+- Containers provide a means to center and horizontally pad your site's contents. Use `.container` for a responsive pixel width or `.container-fluid` for `width: 100%` across all viewport and device sizes.
+- Rows are wrappers for columns. Each column has horizontal `padding` (called a gutter) for controlling the space between them. This `padding` is then counteracted on the rows with negative margins. This way, all the content in your columns is visually aligned down the left side.
+- In a grid layout, content must be placed within columns and only columns may be immediate children of rows.
+- Thanks to flexbox, grid columns without a specified `width` will automatically layout as equal width columns. For example, four instances of `.col-sm` will each automatically be 25% wide from the small breakpoint and up. See the [auto-layout columns](#auto-layout-columns) section for more examples.
+- Column classes indicate the number of columns you'd like to use out of the possible 12 per row. So, if you want three equal-width columns across, you can use `.col-4`.
 - Column `width`s are set in percentages, so they're always fluid and sized relative to their parent element.
 - Columns have horizontal `padding` to create the gutters between individual columns, however, you can remove the `margin` from rows and `padding` from columns with `.no-gutters` on the `.row`.
-- There are five grid tiers, one for each [responsive breakpoint]({{ site.baseurl }}/docs/{{ site.docs_version }}/layout/overview/#responsive-breakpoints): all breakpoints (extra small), small, medium, large, and extra large.
-- Grid tiers are based on minimum widths, meaning they apply to that one tier and all those above it (e.g., `.col-sm-4` applies to small, medium, large, and extra large devices).
-- You can use predefined grid classes or Sass mixins for more semantic markup.
+- To make the grid responsive, there are five grid breakpoints, one for each [responsive breakpoint]({{ site.baseurl }}/docs/{{ site.docs_version }}/layout/overview/#responsive-breakpoints): all breakpoints (extra small), small, medium, large, and extra large.
+- Grid breakpoints are based on minimum width media queries, meaning **they apply to that one breakpoint and all those above it** (e.g., `.col-sm-4` applies to small, medium, large, and extra large devices, but not the first `xs` breakpoint).
+- You can use predefined grid classes (like `.col-4`) or [Sass mixins](#sass-mixins) for more semantic markup.
 
 Be aware of the limitations and [bugs around flexbox](https://github.com/philipwalton/flexbugs), like the [inability to use some HTML elements as flex containers](https://github.com/philipwalton/flexbugs#9-some-html-elements-cant-be-flex-containers).
 
-Sounds good? Great, let's move on to seeing all that in an example.
-
 ## Grid options
 
 While Bootstrap uses `em`s or `rem`s for defining most sizes, `px`s are used for grid breakpoints and container widths. This is because the viewport width is in pixels and does not change with the [font size](https://drafts.csswg.org/mediaqueries-3/#units).