Thread: pgsql: Doc: add comments about PreventInTransactionBlock/IsInTransactio

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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1949135e79e852e19958873203bf32691754faaa

Modified Files
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src/backend/access/transam/xact.c | 8 ++++++++
1 file changed, 8 insertions(+)