Docs: improve warnings about nextval() not producing gapless sequences.

In the documentation for nextval(), point out explicitly that INSERT ...
ON CONFLICT will call nextval() if needed for the insertion case, whether
or not it ends up following the ON CONFLICT path.  This seems to be a
matter of some confusion, cf bug #14126, so let's be clear about it.

Also mention the issue in the CREATE SEQUENCE reference page, since that
is another place where people might expect such things to be covered.

Minor wording improvements nearby, as well.

Back-patch to 9.5 where ON CONFLICT was introduced.

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