pgsql: Document which GUC settings are enums and not strings, along with

Log Message:
-----------
Document which GUC settings are enums and not strings, along with a short
paragraph about what that means.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.196 -> r1.197)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.196&r2=1.197)