pgsql: Document XLOG_INCLUDE_XID a little better

Document XLOG_INCLUDE_XID a little better

I noticed that commit 0bead9af484c left this flag undocumented in
XLogSetRecordFlags, which led me to discover that the flag doesn't
actually do what the one comment on it said it does.  Improve the
situation by adding some more comments.

Backpatch to 14, where the aforementioned commit appears.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/202109212119.c3nhfp64t2ql@alvherre.pgsql

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c1d1ae1db23796e4d1b04f5c087944722cf1446a

Modified Files
--------------
src/backend/access/transam/xloginsert.c | 2 ++
src/include/access/xlog.h               | 2 +-
src/include/access/xlogrecord.h         | 5 +++--
3 files changed, 6 insertions(+), 3 deletions(-)