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