Add contrib function references in the doc index

Bruce Momjian <bruce@momjian.us>

Commit: 8b4c798973033156db58dbca3b605e1045cc587d
Author: Bruce Momjian <bruce@momjian.us>
Date: 2013-07-04T15:33:08Z
Releases: 9.3.0
Add contrib function references in the doc index
Backpatch to 9.3.
Idea from Craig Ringer

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