Add a charset note in "Writing Posts" doc page

There have been a couple of issues recently (#1858 and #1891) that involve browsers rendering some quotation characters improperly due to the `charset` meta value not being defined in the HTML of the layout. This little note seeks to provide some guidance to avoid that issue, and other issues that may crop up around content processor character substitution magic.

(I also removed a space character elsewhere in the page that shouldn't be there.)