debian: Document the postinst triggered invocation in the comment header

debian/changelog

@@ -16,6 +16,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
   * Only trim trailing “/” and “/.” from «dpkg-query --search» arguments if
     they are a pathname, and not a pattern or a substring match.
   * Document dpkg-query --search behavior in man page. Closes: #775379
+  * Document postinst “triggered” argument in debian/dpkg.postinst comment
+    header.
 
  -- Guillem Jover <guillem@debian.org>  Tue, 09 Dec 2014 23:53:18 +0100
 

debian/dpkg.postinst

@@ -4,6 +4,9 @@
 # After the package was installed:
 #	<postinst> configure <old-version>
 #
+# After the package was triggered:
+#	<postinst> triggered <trigger-name>…
+#
 #
 # If prerm fails during upgrade or fails on failed upgrade:
 #	<old-postinst> abort-upgrade <new-version>