update man page

man/dpkg-source.1

@@ -55,6 +55,10 @@ will look for the original source tarfile
 or the original source directory
 .IB directory .orig
 depending on the \fB\-sX\fP arguments.
+
+
+If the source package is being built as a version 3 source package using
+a VCS, no upstream tarball or original source directory is needed.
 .TP
 .BR \-h ", " \-\-help
 Show the usage message and exit.
@@ -162,6 +166,9 @@ will not overwrite existing tarfiles or directories. If this is
 desired then
 .BR \-sA ", " \-sP ", " \-sK ", " \-sU " and " \-sR
 should be used instead.
+.PP
+If the source package is being built as a version 3 source package using
+a VCS, these options do not make sense, and will be ignored.
 .TP
 .BR \-sk
 Specifies to expect the original source as a tarfile, by default