pgsql: Clarify the HINT for 'checkpoint request failed', per recent

Log Message:
-----------
Clarify the HINT for 'checkpoint request failed', per recent complaint
demonstrating that its intent wasn't obvious.

Modified Files:
--------------
pgsql/src/backend/postmaster:
bgwriter.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.23&r2=1.24)