pgsql: Allow binary-coercible types for cast function arguments and

Log Message:
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Allow binary-coercible types for cast function arguments and return types.
Document return type of cast functions.

Also change documentation to prefer the term "binary coercible" in its
present sense instead of the previous term "binary compatible".

Modified Files:
--------------
    pgsql/doc/src/sgml:
        catalogs.sgml (r2.166 -> r2.167)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/catalogs.sgml?r1=2.166&r2=2.167)
        typeconv.sgml (r1.53 -> r1.54)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/typeconv.sgml?r1=1.53&r2=1.54)
    pgsql/doc/src/sgml/ref:
        create_cast.sgml (r1.26 -> r1.27)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_cast.sgml?r1=1.26&r2=1.27)
    pgsql/src/backend/commands:
        functioncmds.c (r1.93 -> r1.94)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c?r1=1.93&r2=1.94)