Fix up the preferred way to specify mw.util.$content

Fix up the preferred way to specify mw.util.$content

- Setting a .mw-body class is the preferred way for $content to be defined
- You can set an extra .mw-body-primary if you use multiple .mw-body classes or for some legacy reasons .mw-body is at a different depth than the content area
- No extensions appear to be using util.$content in a way that would imply they want the body text rather than the overall content area
- Anyone that actually needs the body text can use #mw-content-text.

Change-Id: Ic7b756f139047e4100fcab8f63a968d45ca2b2fe