pgsql: docs: Add short "cover density" description

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.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0b5c0f3bc78d72b753387f84b9188709af3ef8fb

Modified Files
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doc/src/sgml/textsearch.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

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