Обсуждение: pgsql: Document WAL rules related to PD_ALL_VISIBLE in README.

Document WAL rules related to PD_ALL_VISIBLE in README.

Also improve comments.

Discussion: https://postgr.es/m/a50005c1c537f89bb359057fd70e66bb83bce969.camel@j-davis.com
Reviewed-by: Peter Geoghegan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/97c61f70d1b97bdfd20dcb1f2b1be42862ec88c2

Modified Files
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src/backend/access/heap/heapam.c        |  6 ++++--
src/backend/access/heap/visibilitymap.c | 17 +++++++++++------
src/backend/access/transam/README       | 17 +++++++++++++++++
3 files changed, 32 insertions(+), 8 deletions(-)