Improve description of XLOG_RUNNING_XACTS

Hi,

I realized that standby_desc_running_xacts() in standbydesc.c doesn't
describe subtransaction XIDs. I've attached the patch to improve the
description. Here is an example by pg_wlaldump:

* HEAD
rmgr: Standby     len (rec/tot):     58/    58, tx:          0, lsn:
0/01D0C608, prev 0/01D0C5D8, desc: RUNNING_XACTS nextXid 1050
latestCompletedXid 1047 oldestRunningXid 1048; 1 xacts: 1048

* w/ patch
rmgr: Standby     len (rec/tot):     58/    58, tx:          0, lsn:
0/01D0C608, prev 0/01D0C5D8, desc: RUNNING_XACTS nextXid 1050
latestCompletedXid 1047 oldestRunningXid 1048; 1 xacts: 1048; 1
subxacts: 1049

Please review it.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/