Improve documentation about CREATE INDEX CONCURRENTLY.

Improve documentation about CREATE INDEX CONCURRENTLY.

Clarify the description of which transactions will block a CREATE INDEX
CONCURRENTLY command from proceeding, and mention that the index might
still not be usable after CREATE INDEX completes.  (This happens if the
index build detected broken HOT chains, so that pg_index.indcheckxmin gets
set, and there are open old transactions preventing the xmin horizon from
advancing past the index's initial creation.  I didn't want to explain what
broken HOT chains are, though, so I omitted an explanation of exactly when
old transactions prevent the index from being used.)

Per discussion with Chris Travers.  Back-patch to all supported branches,
since the same text appears in all of them.