Explain why .tooltipped should be used sparingly

[muan]

This adds a note on why the tooltip pattern should be used carefully and sparingly.

Our team is planning to add a linting counter for .tooltipped usage to discourage it, and we would like to point people to this documentation.

We now have over 700 tooltips in github/github. There has been many cases where tooltips negatively affect accessibility by either overriding existing content or adding extraneous information that a screen reader user would have to listen to.

The counter we plan to add will not ban additional .tooltipped instances; it would link people to this doc and simply ask them to consider, and increment the tooltip counter if necessary, for the linter to pass.

/cc @primer/ds-core
/cc @dgraham

[emplums]

Yessssss, thanks for writing this up @muan! I am personally excited about a tooltip counter/lint in .com! Tooltips are such a pain when it comes to accessibility!

[shawnbot]

FYI @muan, the merge conflict here stems from the reorganization in #671, and should merge cleanly locally.