documented that wxFileModificationTime() returns 0 (and not a more natural -1) if an error occurs

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@34711 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/function.tex

@@ -939,6 +939,8 @@ Returns true if the file exists and is a plain file.
 
 Returns time of last modification of given file.
 
+The return value is $0$ if an error occured (e.g. file not found).
+
 
 \membersection{::wxFileNameFromPath}\label{wxfilenamefrompath}