BUG #8813: Clarifying OVERLAPS in the Docs

The following bug has been logged on the website:

Bug reference:      8813
Logged by:          Jonathan S. Katz
Email address:      jonathan.katz@excoventures.com
PostgreSQL version: 9.3.2
Operating system:   N/A
Description:

This was reported by Adam Mackler (mackler) in the doc comments, and am
relaying this. My edits are in []

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Section 9.9 (v. 9.3 manual) says "the SQL OVERLAPS operator is supported."

There is no entry in the [documentation] index for "OVERLAPS."  I've been
operating for some time now on the mistaken assumption that OVERLAPS was not
supported, simply because I assumed that if it were, then there would be an
index entry for it.

I propose modifying the [documentation] index by adding an "OVERLAPS" entry.