Documentation accuracy

Background

0.6.0 has some documentation improvements we should make.

What to change

• 1) unclear use case for subscriptions.validate() -> Make this private
• 2) missing Recommendation.includes - Standardize the recommendation interactions. From a documentation perspective this means adding includes. Check that the includes field is also returned when calling EWE.subscriptions.getRecommendations()
• 3) addonInfo can be undefined for Manifest v2 (presumably only applies to addonInfo.bundledSubscriptions and addonInfo.bundledSubscriptionsPath) -> AddonInfo has some defaults for name and version. The individual properties that relate to subscriptions cannot be undefined for MV3 though. Add a note about this.
• 4) FirstRunInfo.warnings can be undefined -> Fix this.
• 5) duplicate documentation for Recommendation.language -> Fix this.
• 6) duplicated documentation for return type of start() -> Fix this.