bpo-38862: IDLE Strip Trailing Whitespace fixes end newlines  (GH-17366)

Extra newlines are removed at the end of non-shell files. If the file only has newlines after stripping other trailing whitespace, all are removed, as is done by patchcheck.py.
diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index 0bd248c..273b583 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -199,7 +199,8 @@
 Strip trailing whitespace
    Remove trailing space and other whitespace characters after the last
    non-whitespace character of a line by applying str.rstrip to each line,
-   including lines within multiline strings.
+   including lines within multiline strings.  Except for Shell windows,
+   remove extra newlines at the end of the file.
 
 .. index::
    single: Run script