pgsql: Add comment to explain why O_EXCL and O_TRUNC can be ignored in

Log Message:
-----------
Add comment to explain why O_EXCL and O_TRUNC can be ignored in
openFlagsToCreateFileFlags() in certain cases.

Modified Files:
--------------
pgsql/src/port:
open.c (r1.18 -> r1.19)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/open.c.diff?r1=1.18&r2=1.19)