man: Clarify how to revert to the serial dpkg-buildpackage behavior

Explicitly mention in dpkg-buildpackage(1) that setting the number
of parallel jobs to 1, restores the serial behavior.

Prompted-by: Thorsten Glaser <t.glaser@tarent.de>

debian/changelog

@@ -21,6 +21,9 @@ dpkg (1.18.14) UNRELEASED; urgency=medium
   * Documentation:
     - Improve the DEB_BUILD_OPTIONS description in dpkg-buildpackage(1).
       And mention the parallel option there.
+    - Explicitly mention in dpkg-buildpackage(1) that setting the number
+      of parallel jobs to 1, restores the serial behavior.
+      Prompted by Thorsten Glaser <t.glaser@tarent.de>.
   * Test suite:
     - Do not fail tests on missing fakeroot, just skip them.
   * Build system:

man/dpkg-buildpackage.man

@@ -254,7 +254,8 @@ to the \fB\-j\fP option except that it does not set the \fBMAKEFLAGS\fP
 environment variable, and as such it is safer to use with any package
 including those that are not parallel-build safe.
 
-\fBauto\fP is the default behavior (since dpkg 1.18.11).
+\fBauto\fP is the default behavior (since dpkg 1.18.11). Setting the number
+of jobs to 1 will restore a serial behavior.
 .TP
 .BR \-D ", " \-\-check\-builddeps
 Check build dependencies and conflicts; abort if unsatisfied (long option