[3.6] bpo-314777: IDLE - improve rstrip entry in doc (GH-3602) (#3605)

'Strip trailing whitespace' is not limited to spaces.  Wording caters to beginners who
do know know the meaning of 'whitespace'.  Multiline string literals are not skipped.
(cherry picked from commit ff70289)

Doc/library/idle.rst

@@ -190,7 +190,9 @@ Format Paragraph
    paragraph will be formatted to less than N columns, where N defaults to 72.
 
 Strip trailing whitespace
-   Remove any space characters after the last non-space character of a line.
+   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.
 
 .. index::
    single: Run script

Lib/idlelib/help.html

@@ -227,7 +227,9 @@ community is 4 spaces.</dd>
 multiline string or selected line in a string.  All lines in the
 paragraph will be formatted to less than N columns, where N defaults to 72.</dd>
 <dt>Strip trailing whitespace</dt>
-<dd>Remove any space characters after the last non-space character of a line.</dd>
+<dd>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.</dd>
 </dl>
 </div>
 <div class="section" id="run-menu-editor-window-only">
@@ -779,7 +781,7 @@ also used for testing.</p>
     The Python Software Foundation is a non-profit corporation.
     <a href="https://www.python.org/psf/donations/">Please donate.</a>
     <br />
-    Last updated on Sep 12, 2017.
+    Last updated on Sep 15, 2017.
     <a href="../bugs.html">Found a bug</a>?
     <br />
     Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.3.6.

Misc/NEWS.d/next/IDLE/2017-09-15-12-38-47.bpo-31477.n__6sa.rst

@@ -0,0 +1,2 @@
+IDLE - Improve rstrip entry in doc. Strip trailing whitespace strips more
+than blank spaces.  Multiline string literals are not skipped.