Doc: add comments about PreventInTransactionBlock/IsInTransactionBlock.

Add a little to the header comments for these functions to make it
clearer what guarantees about commit behavior are provided to callers.
(See commit f92944137 for context.)

Although this is only a comment change, it's really documentation
aimed at authors of extensions, so it seems appropriate to back-patch.

Yugo Nagata and Tom Lane, per further discussion of bug #17434.

Discussion: https://postgr.es/m/17434-d9f7a064ce2a88a3@postgresql.org

src/backend/access/transam/xact.c

@@ -3448,6 +3448,10 @@ AbortCurrentTransaction(void)
  *	a transaction block, typically because they have non-rollback-able
  *	side effects or do internal commits.
  *
+ *	If this routine completes successfully, then the calling statement is
+ *	guaranteed that if it completes without error, its results will be
+ *	committed immediately.
+ *
  *	If we have already started a transaction block, issue an error; also issue
  *	an error if we appear to be running inside a user-defined function (which
  *	could issue more commands and possibly cause a failure after the statement
@@ -3573,6 +3577,10 @@ CheckTransactionBlock(bool isTopLevel, bool throwError, const char *stmtType)
  *	a transaction block than when running as single commands.  ANALYZE is
  *	currently the only example.
  *
+ *	If this routine returns "false", then the calling statement is
+ *	guaranteed that if it completes without error, its results will be
+ *	committed immediately.
+ *
  *	isTopLevel: passed down from ProcessUtility to determine whether we are
  *	inside a function.
  */