Document streaming payload inputs

[kyleknap]

Fixes boto/boto3#477

Here are samples of how it looks:

cc @jamesls @JordonPhillips

[jamesls]

Do we define SeekableStream anywhere?

Is it worth just saying it's a file type?

[kyleknap]

I just made up a class name. We can call it file if we want. I just want to make the point that we accept file like objects that must be seekable. Thoughts?

[JordonPhillips]

It's unfortunate that there's no IO base class that is defined by seek support. Maybe call it file and inject something in the doc string about it requiring seek?

[jamesls]

Yeah something like that would work. My main concern is that someone sees SeekableStream, thinks that's some base class/interface they need to implement, and can't find any information about that.

[kyleknap]

Alright how about this one?

[jamesls]

Looks good.