Add flow control sample documentation

[makeyang]

flow control mechanism is a little bit hard to understand. can you guys add some test to demonstrate its usage?

[ejona86]

That seems reasonable. Were you trying to use StreamObserver or ClientCall?

[makeyang]

ClientCall

[buchgr]

I assume you meant an example?

[ejona86]

I assumed it was include an example that demonstrates interacting with flow control.

[hsaliak]

@zhangkun83 could you help with a flow control example using ClientCall? Having this in the javadoc is sufficient.