#5601: clarify that webbrowser is not meant for file names.

Doc/library/webbrowser.rst

@@ -55,6 +55,10 @@ The following functions are defined:
    under many window managers this will occur regardless of the setting of this
    variable).
 
+   Note that on some platforms, trying to open a filename using this function,
+   may work and start the operating system's associated program.  However, this
+   is neither supported nor portable.
+
    .. versionchanged:: 2.5
       *new* can now be 2.