pgsql: Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_STA

Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_START.

Blocking checkpoint phase 2 requires MarkBufferDirty() and
BUFFER_LOCK_EXCLUSIVE; neither suffices by itself. transam/README documents
this, citing SyncOneBuffer(). Update the DELAY_CHKPT_START documentation to
say this. Expand the heap_inplace_update_and_unlock() comment that cites
XLogSaveBufferForHint() as precedent, since heap_inplace_update_and_unlock()
could have opted not to use DELAY_CHKPT_START.

Commit 8e7e672cdaa6bfec85d4d5dd9be84159df23bb41 added DELAY_CHKPT_START to
heap_inplace_update_and_unlock(). Since commit
bc6bad88572501aecaa2ac5d4bc900ac0fd457d5 reverted it in non-master branches,
no back-patch.

Discussion: https://postgr.es/m/20250406180054.26.nmisch@google.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/818013665259d4242ba641aad705ebe5a3e2db8e

Modified Files
--------------
src/backend/access/heap/heapam.c | 14 +++++++++++---
src/include/storage/proc.h | 8 ++++----
2 files changed, 15 insertions(+), 7 deletions(-)