pv: docs - note about use of $shv() in kemi scripts for shvset param [skip ci]

(cherry picked from commit bcc6370cdc00f25ce3d0f29e168c20ca880dcda2)

src/modules/pv/doc/pv_admin.xml

@@ -56,13 +56,18 @@
 	<section id="pv.p.shvset">
 		<title><varname>shvset</varname> (string)</title>
 		<para>
-		Set the value of a shared variable ($shv(name)). The parameter
+		Set the initial value of a shared variable ($shv(name)). The parameter
 		can be set many times.
 		</para>
 		<para>
 		The value of the parameter has the format:
 		_name_ '=' _type_ ':' _value_
 		</para>
+		<para>
+		Note: this is special useful for usage with KEMI scripts, the $shv(...)
+		variables must be defined during Kamailio initialization in order to
+		become available in all worker processes.
+		</para>
 		<itemizedlist>
 			<listitem><para>_name_: shared variable name</para></listitem>