git-annex chapter dances around the concept of a annex-key

[mih]

It is explained in a box, but more frequently the key is referred to as a "checksum" -- which is mostly, but not entirely, correct.

I am pondering to:

• turn any mention of "key" into "annex-key" to make more obvious that this is a special term
• add a glossary item for "annex-key"
• replace mentions of "checksum" with "annex-key", whenever it is really about the identifier aspect and less about the hash
• introduce a "annex-backend" term too -- presently MD5E is labeled a "checksum"