doc: Adds `--quiet` to the common options.

Fixes #1298

doc/manual/command-ref/opt-common-syn.xml

@@ -8,6 +8,9 @@
     <arg choice='plain'><option>-v</option></arg>
   </group>
 </arg>
+<arg>
+  <arg choice='plain'><option>--quiet</option></arg>
+</arg>
 <arg>
   <group choice='plain'>
     <arg choice='plain'><option>--no-build-output</option></arg>

doc/manual/command-ref/opt-common.xml

@@ -75,6 +75,23 @@
 </varlistentry>
 
 
+<varlistentry><term><option>--quiet</option></term>
+
+  <listitem>
+
+  <para>Decreases the level of verbosity of diagnostic messages
+  printed on standard error.  This is the inverse option to
+  <option>-v</option> / <option>--verbose</option>.
+  </para>
+
+  <para>This option may be specified repeatedly.  See the previous
+  verbosity levels list.</para>
+
+  </listitem>
+
+</varlistentry>
+
+
 <varlistentry><term><option>--no-build-output</option> / <option>-Q</option></term>
 
   <listitem><para>By default, output written by builders to standard