pgsql: Fix things so that array_agg_finalfn does not modify or free its

Log Message:
-----------
Fix things so that array_agg_finalfn does not modify or free its input
ArrayBuildState, per trouble report from Merlin Moncure.  By adopting
this fix, we are essentially deciding that aggregate final-functions
should not modify their inputs ever.  Adjust documentation and comments
to match that conclusion.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        xaggr.sgml (r1.37 -> r1.38)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xaggr.sgml?r1=1.37&r2=1.38)
    pgsql/src/backend/executor:
        nodeWindowAgg.c (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeWindowAgg.c?r1=1.5&r2=1.6)
    pgsql/src/backend/utils/adt:
        array_userfuncs.c (r1.30 -> r1.31)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/array_userfuncs.c?r1=1.30&r2=1.31)
        arrayfuncs.c (r1.157 -> r1.158)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/arrayfuncs.c?r1=1.157&r2=1.158)