Tiny update to protocol overview doc. (#26396)

* Tiny update to protocol overview doc.

Clarify client stream closing and link to more detalied HTTP/2 specific doc.

* apply review comments

* add brief HTTP/2 client stream closing mechanism explenation

* reword to differentiate HTTP/2 stream from gRPC message stream

* apply review comments

CONCEPTS.md

@@ -54,10 +54,11 @@ clients and servers. A concrete embedding over HTTP/2 completes the picture by
 fleshing out the details of each of the required operations.
 
 ## Abstract gRPC protocol
-A gRPC call comprises of a bidirectional stream of messages, initiated by the client. In the client-to-server direction, this stream begins with a mandatory `Call Header`, followed by optional `Initial-Metadata`, followed by zero or more `Payload Messages`. The server-to-client direction contains an optional `Initial-Metadata`, followed by zero or more `Payload Messages` terminated with a mandatory `Status` and optional `Status-Metadata` (a.k.a.,`Trailing-Metadata`).
+A gRPC call comprises of a bidirectional stream of messages, initiated by the client. In the client-to-server direction, this stream begins with a mandatory `Call Header`, followed by optional `Initial-Metadata`, followed by zero or more `Payload Messages`. A client signals end of its message stream by means of an underlying lower level protocol. The server-to-client direction contains an optional `Initial-Metadata`, followed by zero or more `Payload Messages` terminated with a mandatory `Status` and optional `Status-Metadata` (a.k.a.,`Trailing-Metadata`).
 
 ## Implementation over HTTP/2
-The abstract protocol defined above is implemented over [HTTP/2](https://http2.github.io/). gRPC bidirectional streams are mapped to HTTP/2 streams. The contents of `Call Header` and `Initial Metadata` are sent as HTTP/2 headers and subject to HPACK compression. `Payload Messages` are serialized into a byte stream of length prefixed gRPC frames which are then fragmented into HTTP/2 frames at the sender and reassembled at the receiver. `Status` and `Trailing-Metadata` are sent as HTTP/2 trailing headers (a.k.a., trailers).
+The abstract protocol defined above is implemented over [HTTP/2](https://http2.github.io/). gRPC bidirectional streams are mapped to HTTP/2 streams. The contents of `Call Header` and `Initial Metadata` are sent as HTTP/2 headers and subject to HPACK compression. `Payload Messages` are serialized into a byte stream of length prefixed gRPC frames which are then fragmented into HTTP/2 frames at the sender and reassembled at the receiver. `Status` and `Trailing-Metadata` are sent as HTTP/2 trailing headers (a.k.a., trailers). A client signals end of its message stream by setting `END_STREAM` flag on the last DATA frame.
+For a detailed description see [doc/PROTOCOL-HTTP2.md](doc/PROTOCOL-HTTP2.md).
 
 ## Flow Control
 gRPC uses the flow control mechanism in HTTP/2. This enables fine-grained control of memory used for buffering in-flight messages.