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.

diff --git a/doc/src/sgml/textsearch.sgml b/doc/src/sgml/textsearch.sgml
index 9e78286bb59c93ec1c5f5eece553efd31d487f1f..fd54c4425dc641ab973cb37680df836f9dd041ba 100644 (file)
--- a/doc/src/sgml/textsearch.sgml
+++ b/doc/src/sgml/textsearch.sgml
@@ -885,7 +885,9 @@ SELECT plainto_tsquery('english', 'The Fat & Rats:C');
         ranking for the given document vector and query, as described in
         Clarke, Cormack, and Tudhope's "Relevance Ranking for One to Three
         Term Queries" in the journal "Information Processing and Management",
-        1999.
+        1999.  Cover density is similar to <function>ts_rank</> ranking
+        except that the proximity of matching lexemes to each other is
+        taken into consideration.
        </para>
 
        <para>