Add contrib function references in the doc index

Bruce Momjian <bruce@momjian.us>

Commit: 20a1b9e71b18ba823858597dfd445174451ff5e7
Author: Bruce Momjian <bruce@momjian.us>
Date: 2013-07-04T15:33:11Z
Releases: 9.4.0
Add contrib function references in the doc index
Backpatch to 9.3.
Idea from Craig Ringer

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