man: Document the .changes filename that dpkg-buildpackage generates

Prompted-by: Johannes Schauer <josch@debian.org>

debian/changelog

@@ -8,6 +8,8 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
     - Fix formatting in SOURCE_DATE_EPOCH description in dpkg-deb(5).
     - Improve dpkg-deb --build arguments documentation in dpkg-deb(1).
       Prompted by Johannes Schauer <josch@debian.org>.
+    - Document the .changes filename that dpkg-buildpackage generates in
+      dpkg-buildpackage(1). Prompted by Johannes Schauer <josch@debian.org>.
   * Build system:
     - Add support for profiling perl modules.
     - Clean up compiler and linker automatic flag usage in configure.

man/dpkg-buildpackage.1

@@ -64,6 +64,14 @@ build has been requested with \fB\-\-build\fP or equivalent options).
 .IP \fB6.\fP 3
 It runs the \fBchanges\fP hook and calls \fBdpkg\-genchanges\fP to
 generate a \fB.changes\fP file.
+The name of the \fB.changes\fP file will depend on the type of build and
+will be as specific as necessary but not more;
+for a build that includes \fBany\fP the name will be
+\fIsource-name\fP\fB_\fP\fIsource-version\fP\fB_\fP\fIarch\fP\fB.changes\fP,
+or otherwise for a build that includes \fBall\fP the name will be
+\fIsource-name\fP\fB_\fP\fIsource-version\fP\fB_\fP\fBall.changes\fP,
+or otherwise for a build that includes \fBsource\fP the name will be
+\fIsource-name\fP\fB_\fP\fIsource-version\fP\fB_\fP\fBsource.changes\fP.
 Many \fBdpkg\-buildpackage\fP options are forwarded to
 \fBdpkg\-genchanges\fP.
 .IP \fB7.\fP 3