[COMMITTERS] pgsql: Add comments explaining clauses used by CREATE ROLE but not

[Bruce Momjian]

Log Message:
-----------
Add comments explaining clauses used by CREATE ROLE but not ALTER.

Modified Files:
--------------
    pgsql/src/backend/parser:
        gram.y (r2.510 -> r2.511)
        
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.510&r2=2.511)

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