Add docs of missing GUC to pgtrgm.sgml

be8a7a68 introduced pg_trgm.strict_word_similarity_threshold GUC, but missed
docs for that.  This commit fixes that.

Discussion: https://postgr.es/m/fc907f70-448e-fda3-3aa4-209a59597af0%402ndquadrant.com
Author: Ian Barwick
Reviewed-by: Masahiko Sawada, Michael Paquier
Backpatch-through: 9.6