Editor: when TinyMCE is used, add `autocomplete="off"` to the textarea to prevent problems with missing <p> tags when the user reloads the page or uses Go Back/Go Forward buttons, and when editing the same post from two different windows. The autocomplete can be removed with the `the_editor` filter. Part props xsonic, fixes #27251.

git-svn-id: https://develop.svn.wordpress.org/trunk@27366 602fd350-edb4-49c9-b593-d223f7449a82