man: Document that dpkg-source is sometimes better than dpkg-buildpackage -S

Suggested-by: Johannes Schauer <j.schauer@email.de>

debian/changelog

@@ -128,6 +128,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
   * Make dependency checks fatal for dpkg-buildpackage -S.
   * Move "(default)" annotations in scripts --help output after option
     description.
+  * Document in dpkg-buildpackage(1) that using dpkg-source is sometimes
+    better than -S. Suggested by Johannes Schauer <j.schauer@email.de>.
 
   [ Raphaël Hertzog ]
   * Drop myself from Uploaders.

man/dpkg-buildpackage.1

@@ -106,6 +106,9 @@ packages. Passed to \fBdpkg\-genchanges\fP.
 .B \-S
 Specifies a source-only build, no binary packages need to be made.
 Passed to \fBdpkg\-genchanges\fP.
+Note: if what you want is simply to (re-)build the source package, using
+\fBdpkg\-source\fP is always better as it does not require any build
+dependencies to be installed to be able to call the \fBclean\fP target.
 .TP
 .B \-F
 Specifies a normal full build, binary and source packages will be built