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.