pgsql: Add C comment about why synchronous_commit=off behavior can lose

Log Message:
-----------
Add C comment about why synchronous_commit=off behavior can lose
committed transactions in a postmaster crash.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xact.c (r1.291 -> r1.292)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.291&r2=1.292)