bpo-314777: IDLE - improve rstrip entry in doc (#3602)

'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.

* News blurb.

Author: terryjreedy

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 @@ <h3>25.5.1.3. Format menu (Editor window only)<a class="headerlink" href="#forma
 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 @@ <h3>Navigation</h3>
     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.