Introduce a ms_site_not_found filter to replace NOBLOGREDIRECT.

Move some processing down below the point where we bail if there's no site. Add more documentation.

props jeremyfelt.
fixes #21143, see #27003.


git-svn-id: https://develop.svn.wordpress.org/trunk@27663 602fd350-edb4-49c9-b593-d223f7449a82