man: Clarify that dpkg-buildpackage -jN forces parallel builds

This forces the parallel builds regardless of the packaging or upstream
build systems supporting them.

Closes: #780866

debian/changelog

@@ -135,6 +135,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
     - Say control-field-name instead of control-file-field in dpkg-deb(1).
     - Fix option values and pathname markup in dpkg-deb(1).
     - Use .TQ to separate different but related options in dpkg(1).
+    - Clarify that dpkg-buildpackage -jN forces parallel builds, regardless of
+      the packaging or upstream build systems supporting them. Closes: #780866
   * Output message fixes and improvements:
     - Remove trailing newline from string literal in warning calls.
     - Say “execute” instead of “exec” in Dpkg::Changelog::Parse error message.

man/dpkg-buildpackage.1

@@ -186,7 +186,11 @@ equivalent to the
 option of the same name (since dpkg 1.14.7).
 Will add itself to the \fB\%MAKEFLAGS\fP
 environment variable, which should cause all subsequent make
-invocations to inherit the option. Also adds \fBparallel=\fP\fIjobs\fP or
+invocations to inherit the option, thus forcing the parallel setting on
+the packaging (and possibly the upstream build system if that uses make)
+regardless of their support for parallel builds, which might cause build
+failures.
+Also adds \fBparallel=\fP\fIjobs\fP or
 \fBparallel=\fP to the \fBDEB_BUILD_OPTIONS\fP environment variable which
 allows debian/rules files to use this information for their own purposes.
 The \fB\%parallel=\fP\fIjobs\fP or \fBparallel=\fP option in