Fix prose description for where doc-footnote is allowed

[mattgarrish]

The text for doc-footnote says it is only for use outside of doc-endnotes, but that was true prior to deprecating doc-endnote.

It should be updated to allow anywhere, but note to see the doc-endnotes role for lists of notes.

[joanmarie]

Related to this, I think some additional clarification in Example 15 is also called for. That a doc-footnote lives inside a listitem (née doc-endnote) might imply that a listitem that lives inside a doc-endnotes section is still somehow "special". My understanding is that it is not special.

Either way, some commentary about why there are doc-footnotes there, and if they have to be there, might be helpful.

[aleventhal]

Here's the proposed change for Chromium:
https://chromium-review.googlesource.com/c/chromium/src/+/3069120
"Remap list items inside endnote or bibliography sections"

[mattgarrish]

Closing as this was fixed with #39.