Expand the ImageTask documentation

[kean]

Documentation only — no behavior changes, and nothing added to the CHANGELOG.

ImageTask is the type users hold the most, and its most important caveats were either missing or written where DocC never publishes them.

• events. The explanation of how subscription works lived on the private makeStream(), so it was invisible to anyone reading the docs. It now sits on the public property, along with the part that wasn't written down anywhere: every access creates a new independent stream. Reading task.events twice gives you two streams, and iterating both progress and previews creates two subscriptions. Adds a short for await example and a note that a stream buffers, so a slow consumer never misses an event.
• image and response. Both documented only ImagePipeline.Error.cancelled, which read as if cancellation were the only failure mode. They throw any ImagePipeline.Error — loading, decoding, or processing. response also now states that cancelling the awaiting Swift task cancels the ImageTask itself (previously discoverable only from isCancelled's doc) and that awaiting it repeatedly is safe.
• Class-level doc. Adds a usage example: create the task, observe progress, await image.
• cancel(). The public doc gains the outcome (the task fails with .cancelled, streams end with the matching .finished event, and calling it twice or after the task finished does nothing). Inside, // Make sure it gets called once (expensive) never said what was expensive; it now names the actor hop and the teardown it triggers.
• progress and previews. One line each pointing at events for the subscription semantics.

To test

1. swift build — builds clean.
2. Read through Sources/Nuke/ImageTask.swift; the diff is comments only.