- big rewrite of the Sequence documentation:

- big rewrite of the Sequence documentation:

1. Sequence should be associated with MetaData always,
except in the really weird case someone is sharing a Sequence
among multiple metadatas.  Make this a "best practice", end the
confusion of #3951, #3979

2. "optional" is not a thing people use, trim this way down

3. remove confusing language

Change-Id: Iab5aec319da2582092fe2615ee50430f76441aff
(cherry picked from commit a1dcf23e371695d8d3be2e1d7875fad10f16a656)