editorconfig / doc_src: trim trailing whitespace

Commit 0893134543 (Added .editorconfig file (#3332) (#3313),
2016-08-25) trimmed trailing whitespace  for Markdown file (which do
have significant trailing whitespace) but ReStructuredText does not,
and none of our Markdown files cares about this, so let's clean up
whitespace always.

doc_src/cmds/string-shorten.rst

@@ -89,7 +89,7 @@ See Also
 .. BEGIN SEEALSO
 
 - :doc:`string pad <string-pad>` does the inverse of this command, adding padding to a specific width instead.
-  
+
 - The :doc:`printf <printf>` command can do simple padding, for example ``printf %10s\n`` works like ``string pad -w10``.
 
 - :doc:`string length <string-length>` with the ``--visible`` option can be used to show what fish thinks the width is.