Comment on need to MarkBufferDirty() if omitting DELAY_CHKPT_START.

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