[DOC beta] Update RSVP.js documentation.

This brings the vendored version of RSVP up to date with the
documentation changes made in https://github.com/tildeio/rsvp.js/pull/184.

------------

Fix list in `Promise.cast` documentation.

Before:

![screenshot](https://www.monosnap.com/image/OBr2UV08Mi8KSCAPne3TNA1kU.png)

After:

![screenshot](https://www.monosnap.com/image/NS4P8uTrGECtCbK4zK4f4rmLE.png)

YUIDoc will only use documentation found within a code block before any
directives. This means that if you start a code block with `@private`
none of the documentation text for that method/property will be in the
generated output.

Example before:

![screenshot](https://www.monosnap.com/image/RnIb3AFAAcTAaqZoenxZmPGDR.png)

Example after:

![screenshot](https://www.monosnap.com/image/9AuWbnfa3RMJgAGMedbfnLLRN.png)
Merge pull request #4034 from rjackson/update_rsvp_docs

[DOC beta] Update RSVP.js documentation.