[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