Follow-up from "Add missing incoming expunge deleted emails option to docs"

Update docs for mailroom so that it's clear that expunging will remove messages marked for deletion (but are still there). 'Deleted' is a bit ambiguous in this context.

The following discussion from !86422 (merged) should be addressed:

• @msedlakjakubowski started a discussion: (+3 comments)

  @bbodenmiller Thanks for creating this Ben! Docs look good to me!

  @cablett Would you mind reviewing and merging?