Clarified description of error handling for shutil.rmtree().

This closes SF patch #569832.

Doc/lib/libshutil.tex

@@ -78,8 +78,9 @@ file type and creator codes will not be correct.
 \begin{funcdesc}{rmtree}{path\optional{, ignore_errors\optional{, onerror}}}
 \index{directory!deleting}
   Delete an entire directory tree.  If \var{ignore_errors} is true,
-  errors will be ignored; if false or omitted, errors are handled by
-  calling a handler specified by \var{onerror} or raise an exception.
+  errors resulting from failed removals will be ignored; if false or
+  omitted, such errors are handled by calling a handler specified by
+  \var{onerror} or, if that is omitted, they raise an exception.
 
   If \var{onerror} is provided, it must be a callable that accepts
   three parameters: \var{function}, \var{path}, and \var{excinfo}.