Add info about context for Encapsulated tasks docs.

[esbanarango]

When you have an Encapsulated task and you're using ember-concurrency < 2.0.0. You can access the "parent" context using this.owner. This was removed in the new versions and the docs does not mention anything about this.context for the new versions.

#401

[maxfierke]

Neither this.context or this.owner were considered part of the public API, but in v0/v1 they were both set to the same thing. Most folks used this.context in encapsulated tasks, so that's the one that was restored. this.owner was confusing because it does not align with what a "owner" typically is in an Ember app and does not exist in v2.

As mentioned in #401, this.context is still not considered part of the public API and won't be documented, but likely will be made accessible via some new, blessed public API in the near-ish future (at which point this.context may disappear)