pgsql: Move visibility map memory ordering comments.

Move visibility map memory ordering comments.

nodeIndexonlyscan.c has long carried a detailed explanation of why it is
safe for index-only scans to test a visibility map bit without locking
the visibility map buffer, despite the possibility of reading a stale
value. Move that explanation into visibilitymap.c.

This is preparation for an upcoming commit that will move VM checks
needed by index-only scans out of the executor and into heapam.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Suggested-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Discussion: https://postgr.es/m/t6mtqbv2mbfhjni4bvwdgoecppjmxvbyfwl6utovzv76xc2672%40k3o5ryevaeqv

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/903d9d12cc0adcd9a7f92c7b35333e9e40756fbf

Modified Files
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src/backend/access/heap/visibilitymap.c | 27 ++++++++++++++++++++++++++-
src/backend/executor/nodeIndexonlyscan.c | 29 ++++-------------------------
2 files changed, 30 insertions(+), 26 deletions(-)