pg_last_xact_replay_timestamp clarification

The attached, applied patch documents that the timestamp returned by
pg_last_xact_replay_timestamp is the time on the _primary_ that the
transaction was generated.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
new file mode 100644
index 8b5c3c5..3106a0f
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** postgres=# SELECT * FROM pg_xlogfile_nam
*** 14219,14225 ****
         <entry><type>timestamp with time zone</type></entry>
         <entry>Get timestamp of last transaction replayed during recovery.
          This is the time at which the commit or abort WAL record for that
!         transaction was generated.
          If no transactions have been replayed during recovery, this function
          returns NULL.  Otherwise, if recovery is still in progress this will
          increase monotonically.  If recovery has completed then this value will
--- 14219,14225 ----
         <entry><type>timestamp with time zone</type></entry>
         <entry>Get timestamp of last transaction replayed during recovery.
          This is the time at which the commit or abort WAL record for that
!         transaction was generated on the primary.
          If no transactions have been replayed during recovery, this function
          returns NULL.  Otherwise, if recovery is still in progress this will
          increase monotonically.  If recovery has completed then this value will