Documentation doesn't explain record states

I can see that records have these four properties:
- isPending
- isRejected
- isRequested
- isResolved

I gather that when `isResolved = true` the `record.content` property is fulfilled and I can render my template. I'm guessing that `isRejected` is true if the load failed. What is the difference between `isPending` and `isRequested`?

Probably all four should be explained in the documentation with some sort of sample template logic. Example:

``` handlebars
{{#if record.isResolved}}
{{#with record.content as |model|}}

{{/with}}
{{else if record.isRejected}}

{{else}}

{{/if}}
```

No documentation file or test is named in the issue. Start by locating the definitions and usage of isPending, isRejected, isRequested, and isResolved, then verify how record.content behaves for each state. Done means the documentation explains all four states and includes a sample Handlebars template covering resolved, rejected, and loading records.

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