Documents invalid use of `attr` for attribute of `id`

There is an assertion that prevents the use of `attr` to define a
attributes named `id`, this adds API documentation of that fact
Merge pull request #1816 from danmcclain/attr-id-doc

Documents invalid use of `attr` for attribute of `id`
Clarify adapter header documentation
Merge pull request #1818 from bmac/rest-adapter-headers

Clarify adapter header documentation
Expands isDeleted documentation

Expands the example to show deletion states
Merge pull request #1822 from danmcclain/isDeleted-example

Expands isDeleted documentation
Fix the ActiveModelAdapter @extends docstring

The RESTAdapter properties should show up on the ActiveModelAdapter
api docs page.
Merge pull request #1828 from bmac/fix-active-model-adapter-extends-docstring

Fix the ActiveModelAdapter @extends docstring