Add a java specific description of manual flow control to the example. (#10223)

* Add a java specific description of manual flow control to the example.

examples/src/main/java/io/grpc/examples/manualflowcontrol/README.md

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+gRPC Manual Flow Control Example
+=====================
+Flow control is relevant for streaming RPC calls.
+
+By default, gRPC will handle dealing with flow control. However, for specific
+use cases, you may wish to take explicit control.
+
+The default, if you do not disable auto requests, is for the gRPC framework to
+automatically request 1 message at startup and after each onNext call,
+request 1 more. With manual flow control, you explicitly do the request. Note
+that acknowledgements (which is what lets the server know that the receiver can
+handle more data) are sent after an onNext call. The onNext method is called
+when there is both an undelivered message and an outstanding request.
+
+The most common use case for manual flow control is to avoid requiring your
+asynchronous onNext method to block when processing of the read message is being
+done somewhere else.
+
+Another, minor use case for manual flow control, is when there are lots of small
+messages and you are using Netty. To avoid switching back and forth between the
+application and network threads, you can specify a larger initial value (such
+as 5) so that the application thread can have values waiting for it rather than
+constantly having to block and wait for the network thread to provide the next
+value.
+
+### Outgoing Flow Control
+
+The underlying layer (such as Netty) will make the write wait when there is no
+space to write the next message. This causes the request stream to go into
+a not ready state and the outgoing onNext method invocation waits. You can
+explicitly check that the stream is ready for writing before calling onNext to
+avoid blocking. This is done with `CallStreamObserver.isReady()`. You can
+utilize this to start doing reads, which may allow
+the other side of the channel to complete a write and then to do its own reads,
+thereby avoiding deadlock.
+
+### Incoming Manual Flow Control
+
+An example use case is, you have a buffer where your onNext places values from
+the stream. Manual flow control can be used to avoid buffer overflows. You could
+use a blocking buffer, but you may not want to have the thread being used by
+onNext block.
+
+By default, gRPC will configure a stream to request one value at startup and
+then at the completion of each "onNext" invocation requests one more message.
+You can take control of this by disabling AutoRequest on the
+request stream. If you do so, then you are responsible for asynchronously
+telling the stream each time that you would like a new message to be
+asynchronously sent to onNext when one is available. This is done by calling a
+method on the request stream to request messages (while this has a count,
+generally you request 1). Putting this request at the end of your onNext method
+essentially duplicates the default behavior.
+
+#### Client side (server or bidi streaming)
+
+In the `ClientResponseObserver.beforeStart` method, call
+`requestStream.disableAutoRequestWithInitial(1)`
+
+When you are ready to begin processing the next value from the stream call
+`requestStream.request(1)`
+
+#### Server side (client or bidi streaming)
+
+In your stub methods supporting streaming, add the following at the top
+
+1. cast `StreamObserver<> responseObserver`
+   to `ServerCallStreamObserver<> serverCallStreamObserver`
+1. call `serverCallStreamObserver.disableAutoRequest()`
+
+When you are ready to begin processing the next value from the stream call
+`serverCallStreamObserver.request(1)`
+
+### Related documents
+Also see [gRPC Flow Control Users Guide][user guide]
+
+ [user guide]: https://grpc.io/docs/guides/flow-control