defined how this used to work with HTML

We decided to drop name and icons. Adding this in case we need it later
or in packaged apps.
Removed name and icon (closes #97)
removed text about obtaining a manifest

removed text that should be in packaged web apps spec
Editorial cleanup + added DisplayMode enum (closes #134, #133, #117)

* added support for minimal-ui, browser, standalone (closes #117)
* reworked the examples (#137)
* API can only be used from top-level doc (#141)
API only works from top-level docs (closes #141)
Merge pull request #143 from w3c/fallback

Reworked the spec to close all remaining issues.