deb-substvars(5): list fields where substvars do not work

debian/changelog

@@ -33,6 +33,8 @@ dpkg (1.15.4) UNRELEASED; urgency=low
   * Fix dpkg's preinst in case /var/lib/dpkg/alternatives contains unexpected
     sub-directories. Closes: #535138
   * Upgrade Standards-Version to 3.8.2 (no changes).
+  * Update deb-substvars(5) to list fields that do not support substvars.
+    Closes: #535353
 
   [ Modestas Vainius ]
   * Provide a meaningful label for dpkg-gensymbols diff.

man/deb-substvars.5

@@ -31,6 +31,11 @@ string
 .B $
 sign.
 
+While variable substitution is done on all control fields, some of those
+fields are used and needed during the build when the substitution did not
+yet occur. That's why you can't use variables in the \fBPackage\fP,
+\fBSource\fP and \fBArchitecture\fP fields.
+
 Variables can be set using the
 .B \-V
 common option. They can be also specified in the file