Update Fetcher documentation

[FrancoisBlavoet]

The Fetcher documentation needs to be updated for the new Glide 4 API

[googlebot]

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[FrancoisBlavoet]

Unrelated : should glide use @workerthread @mainthread annotations ?
That would make its API simpler to grok

[TWiStErRob]

Yes, I support threading annotations, it helps a lot to know if you can do I/O and other stuff safely. All the components of the flow should be annotated. e.g. I never know if Transcoder is on UI or not. Annotations are more prevalent than the * <p> This will always be called on comments. Adding UI thread annotation on the listener/target interface will break a lot of people's builds (lint), in a good way!

[googlebot]

CLAs look good, thanks!

[sjudd]

Great thanks!