[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?
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.
[DOC beta] Fix Ember.computed docs
Merge pull request #3968 from tricknotes/fix-computed-doc

[DOC beta] Fix Ember.computed docs
[DOC beta] Fix syntax highlight
Merge pull request #3969 from tricknotes/code-highlight

[DOC beta] Fix syntax highlight
Fixes typo in error method documentation.
Merge pull request #3974 from abrinkman/master

Fixes typo in method documentation.
Cleanup test using Ember.$.trim

`String#replace(/\s+/g, '')` is the same as `Ember.$.trim`.
To simplify test code, I rewrite with the latter.
[DOC beta] Fix code highlight and add missing YUIDoc markup