Update Add-on Manifest spec descriptions (#14754)
Summary of the issue:
The add-on datastore has certain expectations of an add-on manifest.
These expectations are not required to work with NVDA, but require manifests to be updated to be hosted on the add-on datastore.
We may want to make these changes in a future API breaking release, and add validation to enforce them.
Description of user facing changes
None
Description of development approach
Updates the add-on manifest spec to match the requirements of the add-on store better.
We may want to add validation for these requirements in a future API breaking release.
