Improve levenshtein() docs.

Fix chars-vs-bytes confusion here too.  Improve poor grammar and
markup.

doc/src/sgml/fuzzystrmatch.sgml

@@ -106,15 +106,20 @@ levenshtein_less_equal(text source, text target, int max_d) returns int
 
   <para>
    Both <literal>source</literal> and <literal>target</literal> can be any
-   non-null string, with a maximum of 255 bytes.  The cost parameters
+   non-null string, with a maximum of 255 characters.  The cost parameters
    specify how much to charge for a character insertion, deletion, or
    substitution, respectively.  You can omit the cost parameters, as in
    the second version of the function; in that case they all default to 1.
-   <literal>levenshtein_less_equal</literal> is accelerated version of
-   levenshtein function for low values of distance. If actual distance
-   is less or equal then max_d, then <literal>levenshtein_less_equal</literal>
-   returns accurate value of it. Otherwise this function returns value
-   which is greater than max_d.
+  </para>
+
+  <para>
+   <function>levenshtein_less_equal</function> is an accelerated version of the
+   Levenshtein function for use when only small distances are of interest.
+   If the actual distance is less than or equal to <literal>max_d</>,
+   then <function>levenshtein_less_equal</function> returns the correct
+   distance; otherwise it returns some value greater than <literal>max_d</>.
+   If <literal>max_d</> is negative then the behavior is the same as
+   <function>levenshtein</function>.
   </para>
 
   <para>