Fix primary status update WAL position description

Hi,

Commit 5f3a49abf7f changed the primary status update message to report the
end of the last inserted WAL record instead of the WAL write position.
But, the protocol docs still describes the WAL position field as
the latest WAL write position, which no longer matches the message sent by
walsender.

Attached patch updates the description of the WAL position field to
indicate the end of the
last inserted WAL record, matching the implementation.

Regards,

--
Fujii Masao