Update manifest documentation to Crosswalk 9 and later

All Crosswalk channels now support the W3C manifest v1, so update the documentation in the following ways:

- remove the drop-down selector filtering manifest fields based on Crosswalk version and platform. We don't need to document anything earlier than the stable channel
- remove any mention of the previous Crosswalk versions from the various pages
- update examples to reflect the W3C manifest
- add one section about "other fields" (xwalk_version and xwalk_description)

Once this is merged I will update the sample apps accordingly.