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/emit.rst

@@ -27,7 +27,7 @@ The following code first defines an event handler for the generic event named 't
     function event_test --on-event test_event
         echo event test: $argv
     end
-    
+
     emit test_event something