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.
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