man: Document that dpkg sets DPKG_ADMINDIR on maintainer scripts environment

debian/changelog

@@ -11,6 +11,7 @@ dpkg (1.18.17) UNRELEASED; urgency=medium
   * Documentation:
     - Clarify that dpkg-buildpackage does not run «dpkg-source --before-build»
       when using the -T option. Closes: #649531
+    - Document that dpkg sets DPKG_ADMINDIR on maintainer script environment.
   * Packaging:
     - Do not let dh_perl insert a dependency for libdpkg-perl, because it
       emits perl instead of perl:any. Instead hardcode a perl:any dependency

man/dpkg.man

@@ -902,6 +902,11 @@ this variable empty, but if \fB\-\-force\-script\-chrootless\fP is
 specified then the \fBchroot\fP(2) call is skipped and \fBinstdir\fP
 is non-empty.
 .TP
+.B DPKG_ADMINDIR
+Defined by \fBdpkg\fP on the maintainer script environment to indicate
+the \fBdpkg\fP administrative directory to use (since dpkg 1.16.0).
+This variable is always set to the current \fB\-\-admindir\fP value.
+.TP
 .B DPKG_SHELL_REASON
 Defined by \fBdpkg\fP on the shell spawned on the conffile prompt to
 examine the situation (since dpkg 1.15.6).