Add contrib module functions to docs' function index

Hi all

Andreas Heiduk on -bugs suggested that we add the functions provided by
contrib modules to the function index in the docs, so it's easier to go
from, say,  "what the heck is idx(...)" to finding it in the intarray
contrib module.

This seems like a good idea and I'd like to pop it in the TODO until I
get time to check it out, flagged as a minor/newbie-friendly problem.
Any objections?

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services