pgsql: Mark default_transaction_read_only as GUC_REPORT.

Mark default_transaction_read_only as GUC_REPORT.

This allows clients to find out the setting at connection time without
having to expend a query round trip to do so; which is helpful when
trying to identify read/write servers. (One must also look at
in_hot_standby, but that's already GUC_REPORT, cf bf8a662c9.)
Modifying libpq to make use of this will come soon, but I felt it
cleaner to push the server change separately.

Haribabu Kommi, Greg Nancarrow, Vignesh C; reviewed at various times
by Laurenz Albe, Takayuki Tsunakawa, Peter Smith.

Discussion: https://postgr.es/m/CAF3+xM+8-ztOkaV9gHiJ3wfgENTq97QcjXQt+rbFQ6F7oNzt9A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d16f8c8e416d288bd4734ed5f14076b62ec8d153

Modified Files
--------------
doc/src/sgml/libpq.sgml | 4 +++-
doc/src/sgml/protocol.sgml | 4 +++-
src/backend/utils/misc/guc.c | 3 ++-
3 files changed, 8 insertions(+), 3 deletions(-)