dialog: docs -remark wabout to-tag parameter for dlg_get()

(cherry picked from commit b498a0ad470a83f1e21c45a585926509827dda50)

src/modules/dialog/doc/dialog_admin.xml

@@ -2081,7 +2081,8 @@ dlg_bridge("sip:[email protected]", "sip:[email protected]",
 			</para>
 		</listitem>
 		<listitem>
-			<para><emphasis>ttag</emphasis> - SIP To tag.
+			<para><emphasis>ttag</emphasis> - SIP To tag (it can be empty string
+			to match a dialog during call setup (eg., match for CANCEL)).
 			</para>
 		</listitem>
 		</itemizedlist>