Added documentation to `Encoder` and its `flush` function.
Added documentation to `Encoder` and its `flush` function.

davll/gfx,Bastacyclop/gfx,gfx-rs/gfx-rs,JohnColanduoni/gfx,sectopod/gfx-rs,ColinKinloch/gfx,tanderegg/gfx,JohnColanduoni/gfx,tanderegg/gfx,ColinKinloch/gfx