Update the documentation to the sqlite3_set_auxdata() and sqlite3_get_auxdata()

Update the documentation to the sqlite3_set_auxdata() and sqlite3_get_auxdata()
routines to make it clear that they do not work as one might expect when they
are called during query planning, instead of during query execution.  The JSON
routines misuse those interfaces, so add a special flag to JSON routines that
prevents them from being invoked during query planning.  Fix for the problem
in [forum:/forumpost/a655ee159eca1ea5|forum post a655ee159eca1ea5].

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