Update TODO.md

* Minor language changes that are hopefully clearer.
* Added "Learning DI" to the "Broken docs" section as requested in the https://github.com/zendframework/zf2-documentation/issues/703 issue. 

Can Zend\Session be removed from the list?
Merge branch 'patch-1' of git://github.com/Ore4444/zf2-documentation into Ore4444-patch-1
Added ref to zf2-docs issue tracker
Merge branch 'Ore4444-patch-1'

Merging #714 https://github.com/zendframework/zf2-documentation/pull/714