pgsql: Add some comments about why function parameter default

Log Message:
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Add some comments about why function parameter default expressions are
restricted.

Modified Files:
--------------
pgsql/src/backend/commands:
functioncmds.c (r1.106 -> r1.107)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c?r1=1.106&r2=1.107)