Various documentation improvements. Add a lot of index entries for

the builtin functions (although some more entries are still needed),
and remove the duplicate index entries that have been causing
collateindex.pl warnings. Consistently use "int" and "bigint", rather
than a mix of "int", "integer", "int4", "bigint", and "int8". Make
parenthesis style in syntax examples more consistent. Various
copy-editing for newly-added documentation and SGML markup fixes.