Fix property name in code comment

The property name of `Ember.Array#hasArrayObservers` is incorrect.
Remove unexist param's comment

`Ember.Enumerable#enumerableContentDidChange` allow to be given two arguments.
* `removing`
* `adding`

Not contains `start`.
[beta] better docs for modelFor

I wanted to stress that its main utility was for
accessing models on parent routes rather than
giving the impression that it's useful/meaningful
for grabbing models from any ol route, even if
it's not in the current route hierarchy, which
doesn't really make sense, right?
[beta] Remove unused variable in test
Merge pull request #3959 from pangratz/cleanup_test

Remove unused variable in test
Merge pull request #3957 from tricknotes/fix-code-comment

Fix code comment
Merge pull request #3958 from machty/modelfor-doc

[beta] better docs for modelFor
Fix to highlight code in document
Merge pull request #3960 from tricknotes/fix-to-highlight-code-in-document

Fix to highlight code in document
[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.