Add a design doc (#2079)

Motivation:

For library maintainers and some users, having an understanding of how
the library is structured is incredibly helpful.

Modifications:

- Add a high-level design doc linking to the relevant symbols
- Fix up a couple of other DocC warnings

Result:

More docs

---------

Co-authored-by: Gus Cairo <[email protected]>

Author: glbrntt

Sources/GRPCCore/Call/Client/ClientRequest.swift

@@ -21,7 +21,7 @@
 /// See ``StreamingClientRequest`` for streaming requests and ``ServerRequest`` for the
 /// servers representation of a single-message request.
 ///
-/// ## Creating ``Single`` requests
+/// ## Creating requests
 ///
 /// ```swift
 /// let request = ClientRequest<String>(message: "Hello, gRPC!")

Sources/GRPCCore/Call/Client/ClientResponse.swift

@@ -29,7 +29,7 @@
 /// an ``RPCError`` describing why the RPC failed, including an error code, error message and any
 /// metadata sent by the server.
 ///
-/// ### Using ``Single`` responses
+/// ### Using responses
 ///
 /// Each response has a ``accepted`` property which contains all RPC information. You can create
 /// one by calling ``init(accepted:)`` or one of the two convenience initializers:
@@ -153,7 +153,7 @@ public struct ClientResponse<Message: Sendable>: Sendable {
 /// to execute the request. The failure case contains an ``RPCError`` describing why the RPC
 /// failed, including an error code, error message and any metadata sent by the server.
 ///
-/// ### Using ``Stream`` responses
+/// ### Using streaming responses
 ///
 /// Each response has a ``accepted`` property which contains RPC information. You can create
 /// one by calling ``init(accepted:)`` or one of the two convenience initializers:

Sources/GRPCCore/Call/Server/ServerResponse.swift

@@ -28,7 +28,7 @@
 /// of the RPC failed. The failure case contains an ``RPCError`` describing why the RPC failed,
 /// including an error code, error message and any metadata sent by the server.
 ///
-/// ### Using ``Single`` responses
+/// ### Using responses
 ///
 /// Each response has an ``accepted`` property which contains all RPC information. You can create
 /// one by calling ``init(accepted:)`` or one of the two convenience initializers:
@@ -144,7 +144,7 @@ public struct ServerResponse<Message: Sendable>: Sendable {
 /// contains an ``RPCError`` describing why the RPC failed, including an error code, error
 /// message and any metadata to send to the client.
 ///
-/// ### Using ``Stream`` responses
+/// ### Using streaming responses
 ///
 /// Each response has an ``accepted`` property which contains all RPC information. You can create
 /// one by calling ``init(accepted:)`` or one of the two convenience initializers: