Document that WAIT FOR LSN is timeline-blind

Document that WAIT FOR LSN is timeline-blind

WAIT FOR LSN compares only the numeric LSN and has no notion of which
timeline a WAL record belongs to.  There are many possible scenarios when
timeline-switching can break read-your-writes consistency.  The proper
analysis and timeline support is possible in the next major release.  Yet
just document the current behaviour.

Reported-by: Xuneng Zhou <xunengzhou@gmail.com>
Author: Alexander Korotkov <aekorotkov@gmail.com>