Update manual.

Author: wolfpld

manual/tracy.tex

@@ -3320,7 +3320,7 @@ \subsection{Compare traces window}
 
 \subsubsection{Source files diff}
 
-To see what changes were made in the source code between the two compared traces, select the \emph{Source diff} compare mode. This will display a list of deleted, added, and changed files.
+To see what changes were made in the source code between the two compared traces, select the \emph{Source diff} compare mode. This will display a list of deleted, added, and changed files. By default, the difference is calculated from the older trace to the newer one. You can reverse this by clicking on the \emph{Switch} button.
 
 Please note that changes will be registered only if the file has the same name and location in both traces. Tracy does not resolve file renames or moves.