Merge pull request #15821 from jtattermusch/concepts_polish

Improve CONCEPTS.md wording

CONCEPTS.md

@@ -9,8 +9,9 @@ combination of the supported languages.
 
 ## Interface
 
-Developers using gRPC start with the description of an RPC service (a collection
-of methods), and generate client and server side interfaces. The server implements
+Developers using gRPC start with a language agnostic description of an RPC service (a collection
+of methods). From this description, gRPC will generate client and server side interfaces
+in any of the supported languages. The server implements
 the service interface, which can be remotely invoked by the client interface.
 
 By default, gRPC uses [Protocol Buffers](https://github.com/google/protobuf) as the
@@ -41,7 +42,7 @@ asynchronous flavors.
 
 gRPC supports streaming semantics, where either the client or the server (or both)
 send a stream of messages on a single RPC call. The most general case is
-Bidirectional Streaming where a single gRPC call establishes a stream where both
+Bidirectional Streaming where a single gRPC call establishes a stream in which both
 the client and the server can send a stream of messages to each other. The streamed
 messages are delivered in the order they were sent.