Description
As discussed recently in the team slack, this adds an initial explanation of the intention behind the basic .btn class - as a starting point for either using the extra predefined styles, or for an author's own stylings.
Also adds a callout reminding authors to at least define some focus styling if they intend to use it "naked".
Motivation & Context
We're often seeing authors using just .btn, and this "naked" use is not actually documented/explained anywhere. Also, authors then forgetting to create focus styles causes accessibility issues further down the line if they're not expecting it. We may decide in future to still add some absolutely basic focus styling, just in case, but this would be a breaking change for the next minor release/