[doc] update the out-of-source documentation in the buildsystem-howto

doc/buildsystem/buildsystem.tex

@@ -1311,6 +1311,8 @@ entries in the opts file:
 \item \texttt{BUILDDIR} configure the modules to use out-of-source
   build. The binaries are built in subdirectories of each module,
   named according to the \texttt{BUILDDIR} variable.
+  If \texttt{BUILDDIR} is an absolute path (starts with \texttt{'\string\'}),
+  the build directory is set to \texttt{BUILDDIR/module-name} for each module.
 \item \texttt{\textit{COMMAND}\,\_FLAGS}\\ the most important ones are:
   \begin{itemize}
   \item \texttt{AUTOGEN\_FLAGS} specifies options for the