Documentation improvement for pg_trgm

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