pgsql: Add docs of missing GUC to pgtrgm.sgml

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/19dc23a5ef7559cf22cdf586b9b4be7ad4497ba0

Modified Files
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doc/src/sgml/pgtrgm.sgml | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)