Doc: alphabetize the regexp_foo() function descriptions in 9.7.3.

Doc: alphabetize the regexp_foo() function descriptions in 9.7.3.

For no visible reason (other than historical accident no doubt),
regexp_replace() was out of order.  Re-order to match the way
that these functions are listed in 9.4.  (That means substring()
remains first, because it's SQL-standard and the rest aren't.)
I've not touched the text other than to move it.  This is just
to reduce confusion in the diffs for upcoming additions.