mcp: clarify documentation for the streamable transports

After a deep dive with the team, document areas of the codebase
identified as unclear. For now, this is a documentation-only commit (apart
from one switch of jsonrpc2.Request.ID.IsValid->Request.IsCall). Special
TODO(cleanup) comments are added for additional code changes in
follow-up commits.

Also, include high level doc comments for server and client transports,
in new streamable_server.go and streamable_client.go. Eventually, the
server and client transport logic should be migrated to these files, as
streamable.go has gotten too large. However, we can do that at a future
date, to minimize diff.

internal/jsonrpc2/conn.go

@@ -55,9 +55,16 @@ type ConnectionOptions struct {
 }
 
 // Connection manages the jsonrpc2 protocol, connecting responses back to their
-// calls.
-// Connection is bidirectional; it does not have a designated server or client
-// end.
+// calls. Connection is bidirectional; it does not have a designated server or
+// client end.
+//
+// Note that the word 'Connection' is overloaded: the mcp.Connection represents
+// the bidirectional stream of messages between client an server. The
+// jsonrpc2.Connection layers RPC logic on top of that stream, dispatching RPC
+// handlers, and correlating requests with responses from the peer.
+//
+// Some of the complexity of the Connection type is grown out of its usage in
+// gopls: it could probably be simplified based on our usage in MCP.
 type Connection struct {
 	seq int64 // must only be accessed using atomic operations