pgsql: Improve the description of XLOG_RUNNING_XACTS.

Improve the description of XLOG_RUNNING_XACTS.

Previously, the description of XLOG_RUNNING_XACTS showed only
top-transaction XIDs and whether subtransactions overflowed. This commit
improves it to show individual subtransaction XIDs. This also improves the
description of overflowed subtransactions.

This additional information can be helpful for testing and debugging
purposes.

Author: Masahiko Sawada
Reviewd by: Fujii Masao, Kyotaro Horiguchi, Ashutosh Bapat, Bharath Rupireddy
Discussion: https://postgr.es/m/CAD21AoAqvaE+XEeXHHPdAGQPcCoGXxuoeutq_nWhUSQvTt5+tA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/568546f7e46a48c065c631338c0eba1de9264ff1

Modified Files
--------------
src/backend/access/rmgrdesc/standbydesc.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)