docs: Add short "cover density" description

Bruce Momjian <bruce@momjian.us>

Commit: 0b5c0f3bc78d72b753387f84b9188709af3ef8fb
Author: Bruce Momjian <bruce@momjian.us>
Date: 2014-03-24T19:46:59Z
Releases: 9.4.0
docs:  Add short "cover density" description

Also, previous commit 1420f3a9827a39e5c6c998f6fa35a6fc97903145 to fix
ts_rank_cd() for stripped lexemes was from a patch created by Alex Hill.

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doc/src/sgml/textsearch.sgml modified +3 −1