man: Clarify that dpkg-buildpackage -T does not run dpkg-source --before-build

Closes: #649531

debian/changelog

@@ -1,6 +1,9 @@
 dpkg (1.18.17) UNRELEASED; urgency=medium
 
   [ Guillem Jover ]
+  * Documentation:
+    - Clarify that dpkg-buildpackage does not run «dpkg-source --before-build»
+      when using the -T option. Closes: #649531
   * Packaging:
     - Do not let dh_perl insert a dependency for libdpkg-perl, because tying
       the modules to a specific interpreter is wrong.

man/dpkg-buildpackage.man

@@ -165,7 +165,8 @@ Equivalent to \fB\-\-build=full\fP, \fB\-\-build=source,binary\fP or
 .TQ
 .BR \-T ", " \-\-rules\-target= \fItarget\fP[,...]
 Calls \fBdebian/rules\fP \fItarget\fP once per target specified, after
-having setup the build environment, and stops the package build process
+having setup the build environment (except for calling
+\fBdpkg\-source \-\-before\-build\fP), and stops the package build process
 here (since dpkg 1.15.0, long option since dpkg 1.18.8, multi-target
 support since dpkg 1.18.16).
 If \fB\-\-as\-root\fP is also given, then the command is executed