man: Document DPKG_DATADIR in an ENVIRONMENT section in dpkg-architecture(1)

debian/changelog

@@ -1,6 +1,8 @@
 dpkg (1.18.18) UNRELEASED; urgency=medium
 
   [ Guillem Jover ]
+  * Documentation:
+    - Document DPKG_DATADIR in an ENVIRONMENT section in dpkg-architecture(1).
   * Packaging:
     - Revert the perl to perl:any dependency change in libdpkg-perl, as that
       breaks debootstrap in stable (jessie), testing (stretch) and unstable,

man/dpkg-architecture.man

@@ -356,6 +356,12 @@ endif
 In any case, you should never use \fBdpkg \-\-print\-architecture\fP to get
 architecture information during a package build.
 .
+.SH ENVIRONMENT
+.TP
+.B DPKG_DATADIR
+If set, it will be used as the \fBdpkg\fP data directory, where the
+architecture tables are located (since dpkg 1.14.17).
+Defaults to «%PKGDATADIR%».
 .SH NOTES
 All long command and option names available only since dpkg 1.17.17.
 .