[DOC beta] Fix ClassMixin.reopen docs.
[DOC beta] Fix CoreObject.toString docs.
[DOC beta] Fix Ember.TextField document markup
[DOC beta] Fix Ember.Checkbox document markup
Merge pull request #3961 from tricknotes/fix-document-markup

Add newline to fix document markup
Merge pull request #3962 from tricknotes/fix-document-markup2

Add newline to highlight code example in document
Merge pull request #3963 from rjackson/fix_reopen_docs

[DOC beta] Fix ClassMixin.reopen docs.
Merge pull request #3964 from rjackson/fix_toString_docs

[DOC beta] Fix CoreObject.toString docs.