Split the content to balance open tags when `<!--nextpage-->` and `<!--more.*?-->` are used. Needs filter inline docs. The 4 unit tests that were previously failing for ticket 6297 now pass.

See #6297. 
Props devesine.



git-svn-id: https://develop.svn.wordpress.org/trunk@26050 602fd350-edb4-49c9-b593-d223f7449a82