small doc patch for regexp_replace

Is there any reason, why regexp_replace is not included in the tables of the
string functions?

http://developer.postgresql.org/docs/postgres/functions-string.html

The appended patch adds regexp_replace() and links to the pattern matching
section for substring / regexp_replace.

I noticed that in the table the declaration of the arguments is
inconsistent, for example:

lower(string)
ascii(text)
length(string text)

also in the sgml, there is sometimes "text" and sometimes
"<type>text</type>".

I'd fix that if someone told me what the preferred (if any) form is...

Joachim