pgsql: Document that the regexp split functions ignore zero-length

Log Message:
-----------
Document that the regexp split functions ignore zero-length matches in
certain corner cases.  Per discussion, the code does what we want, but
it really needs to be documented that these functions act differently
from regexp_matches.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.384 -> r1.385)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.384&r2=1.385)