pgsql: Minor code clarity improvements in array_agg functions, and add a

Log Message:
-----------
Minor code clarity improvements in array_agg functions, and add a comment
about how this is playing fast and loose with the type system.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
array_userfuncs.c (r1.24 -> r1.25)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/array_userfuncs.c?r1=1.24&r2=1.25)