Add note on APG vs. browser multiselect listbox keyboard pattern

[smhigley]

Fixes #2193

Adds a note with the wording we discussed in a meeting on the Rearrangeable Listbox multiselect example.

[mcking65]

Thank you @smhigley!

Looks great. I just made one minor revision to include a link back to the keyboard interaction section of the pattern where the multiple selection model is defined.

[mcking65]

Since this is a minor editorial revision and I see a thumbs up from @JAWS-test, who raised the original issue, I am merging without further review.

Note: this is not going directly to main. It is will be merged with the restructuring project. I'm doing it this way so we can avoid fixing it twice. The move-examples branch should be in main in a couple weeks or so.