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

doc/src/sgml/pgtrgm.sgml

@@ -257,12 +257,29 @@
     </term>
     <listitem>
      <para>
-      Sets the current word similarity threshold that is used by
+      Sets the current word similarity threshold that is used by the
       <literal>&lt;%</> and <literal>%&gt;</> operators.  The threshold
       must be between 0 and 1 (default is 0.6).
      </para>
     </listitem>
    </varlistentry>
+   <varlistentry id="guc-pgtrgm-strict-word-similarity-threshold" xreflabel="pg_trgm.strict_word_similarity_threshold">
+    <term>
+     <varname>pg_trgm.strict_word_similarity_threshold</varname> (<type>real</type>)
+     <indexterm>
+      <primary>
+       <varname>pg_trgm.strict_word_similarity_threshold</varname> configuration parameter
+      </primary>
+     </indexterm>
+    </term>
+    <listitem>
+     <para>
+      Sets the current strict word similarity threshold that is used by the
+      <literal>&lt;&lt;%</literal> and <literal>%&gt;&gt;</literal> operators.  The threshold
+      must be between 0 and 1 (default is 0.5).
+     </para>
+    </listitem>
+   </varlistentry>
   </variablelist>
  </sect2>