Documenting the behavior of the upload_file function

[gemblerz]

plugin.upload_file() uploads user-specified files to cloud. It is unclear how this works in user application container. We will need to describe the behavior of the function so that users know what to expect from this function.

One question raised on this topic was "I am sending files (~200MB) using the function. When is safe for the application to exit, and the system terminates the application container?" The answer is that the upload_file function copies the file to a permanent storage before returning. So, it is safe to terminate user application after the function returns.

I suggest adding a bit of information about this behavior in the doc.