Documentation improvement for pg_trgm

Documentation of word_similarity() and strict_word_similarity() functions
contains some vague wordings which could confuse users.  This patch makes
those wordings more clear.  word_similarity() was introduced in PostgreSQL 9.6,
and corresponding part of documentation needs to be backpatched.

Author: Bruce Momjian, Alexander Korotkov
Discussion: https://postgr.es/m/20180526165648.GB12510%40momjian.us
Backpatch: 9.6, where word_similarity() was introduced

Author: akorotkov

doc/src/sgml/pgtrgm.sgml

@@ -153,8 +153,8 @@
    This function returns a value that can be approximately understood as the
    greatest similarity between the first string and any substring of the second
    string.  However, this function does not add padding to the boundaries of
-   the extent.  Thus, a whole word match gets a higher score than a match with
-   a part of the word.
+   the extent.  Thus, the number of additional characters present in the
+   second string is not considered, except for the mismatched word boundry.
   </para>
 
   <table id="pgtrgm-op-table">