Explain the commands-in-DUNE_CONTROL_PATH trick a little better.

In particular, the search gets faster _because of_ the use of ls,
not _despite_ of it.

[[Imported from SVN: r6769]]

doc/buildsystem/buildsystem.tex

@@ -1543,17 +1543,19 @@ configuration or to rerun all checks, the cache file should be removed manually.
 
 \minisec{Faster module search}
 If the environment variable \texttt{DUNE\_CONTROL\_PATH} is not set, \dunecontrol 
-recursively searches for \dune modules below the working directory. The search can be
+recursively searches for \dune modules below the working directory. Depending on
+your installation, this search may take quite a bit of time.  It can be
 avoided by including the paths to the \texttt{dune.module} files of all modules in 
 \texttt{DUNE\_CONTROL\_PATH}.
 
+This list of paths can actually be created on the fly by a command.
 Let us assume all \dune modules are in one directory, which is the working directory 
-for \dunecontrol, too. By adding the the line
+for \dunecontrol, too. By adding the line
 \begin{lstlisting}[language=make,showstringspaces=false]
 DUNE_CONTROL_PATH="`ls */dune.module | tr '\n' ':'`"
 \end{lstlisting}
-to your opts file, no search will be performed despite the faster, non-recursive 
-\texttt{ls} command.
+to your opts file, no search will be performed.  Instead, only the faster, non-recursive 
+\texttt{ls} command is called.
 
 \section{Further documentation}