pgsql: Improve the message logged when recovery is paused.

Improve the message logged when recovery is paused.

When recovery target is reached and recovery is paused because of
recovery_target_action=pause, executing pg_wal_replay_resume() causes
the standby to promote, i.e., the recovery to end. So, in this case,
the previous message "Execute pg_wal_replay_resume() to continue"
logged was confusing because pg_wal_replay_resume() doesn't cause
the recovery to continue.

This commit improves the message logged when recovery is paused,
and the proper message is output based on what (pg_wal_replay_pause
or recovery_target_action) causes recovery to be paused.

Author: Sergei Kornilov, revised by Fujii Masao
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/19168211580382043@myt5-b646bde4b8f3.qloud-c.yandex.net

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b0236508d3589a45e574284cd3303b689111765d

Modified Files
--------------
src/backend/access/transam/xlog.c | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)