Revert "Make monodoc 4.5 only it has no 4.0 specific api". It's intentional that monodoc has a 4.0 compiled version too.

Some project (namely MonoDevelop) depends on a 4.0 monodoc.dll version.

This reverts commit 3fa0abaca8deb2609c104888df8039f021d7a4cd.