return 0 (meaning the file is not seekable, as the docs now explain) instead of 4KB for the files in sysfs under Linux (#9965)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@55726 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/file.h

@@ -82,6 +82,12 @@ public:
 
     /**
         Returns the length of the file.
+
+        This method may return wxInvalidOffset if the length couldn't be
+        determined or also 0 even for non-empty files if the file is not
+        seekable. In general, the only way to determine if the file for which
+        this function returns 0 is really empty or not is to try reading from
+        it.
     */
     wxFileOffset Length() const;