Announce when formula has been completely deleted.

Bug: 23224425
Bug: 23695835

This adds a "cleared" announcement when last token/character is deleted.
Also change the existing "clear" announcements to "cleared".

Improve the comments for desc_clr and desc_del to address prior
misunderstandings.

Improve xml comment formatting consistency.

Change-Id: Ibe5197ceac132b73d72f8ace3982238249f5e82b