pgsql: Update obsolete comment: it's no longer the case that mdread()

Log Message:
-----------
Update obsolete comment: it's no longer the case that mdread() will allow
reads beyond EOF, except by special coercion.

Modified Files:
--------------
pgsql/src/backend/storage/buffer:
bufmgr.c (r1.220 -> r1.221)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/bufmgr.c.diff?r1=1.220&r2=1.221)