man: Clarify that dpkg-architecture uses some of its own variables

Closes: #966111

man/dpkg-architecture.pod

@@ -35,13 +35,14 @@ B<dpkg-architecture>
 provides a facility to determine and set the build and
 host architecture for package building.
 
-The build architecture is always determined by an external call to
+The build architecture is always determined by either the B<DEB_BUILD_ARCH>
+variable if set (and B<--force> not being specified) or by an external call to
 B<dpkg>(1), and cannot be set at the command line.
 
 You can specify the host architecture by providing one or both of the options
-B<--host-arch> and B<--host-type>. The default is determined by
-an external call to
-B<gcc>(1),
+B<--host-arch> and B<--host-type>, otherwise the B<DEB_HOST_ARCH> variable
+is used if set (and B<--force> not being specified). The default is
+determined by an external call to B<gcc>(1),
 or the same as the build architecture if B<CC> or gcc are both not
 available. One out of B<--host-arch> and B<--host-type> is
 sufficient, the value of the
@@ -229,8 +230,9 @@ Example paths: /lib/powerpc64le-linux-gnu/, /usr/lib/i386-kfreebsd-gnu/.
 
 =head1 VARIABLES
 
-The following variables are set by B<dpkg-architecture> (see the
-B<TERMS> section for a description of the naming scheme):
+The following variables are read from the environment (unless B<--force>
+has been specified) and set by B<dpkg-architecture> (see the B<TERMS>
+section for a description of the naming scheme):
 
 =over