man: Document DPKG_HOOK_ACTION also in ENVIRONMENT section

debian/changelog

@@ -6,6 +6,7 @@ dpkg (1.18.2) UNRELEASED; urgency=low
     - Fix grammar in dpkg-architecture(1).
       Thanks to Chris Lamb <lamby@debian.org>. Closes: #787616
     - Use the feature area name in the dpkg-buildflags(1) subsection title.
+    - Document DPKG_HOOK_ACTION also in dpkg(1) ENVIRONMENT section.
 
   [ Updated programs translations ]
   * Dutch (Frans Spiesschaert). Closes: #789097

man/dpkg.1

@@ -791,6 +791,11 @@ Defined by \fBdpkg\fP on the shell spawned on the conffile prompt to
 examine the situation (since dpkg 1.15.6).
 Contains the path to the new conffile.
 .TP
+.B DPKG_HOOK_ACTION
+Defined by \fBdpkg\fP on the shell spawned when executing a hook action
+(since dpkg 1.15.4).
+Contains the current \fBdpkg\fP action.
+.TP
 .B DPKG_RUNNING_VERSION
 Defined by \fBdpkg\fP on the maintainer script environment to the
 version of the currently running \fBdpkg\fP instance (since dpkg 1.14.17).