docs: document HTTP 404 / -32601 response for unsupported RPC methods

Author: kurtisvg

docs/specification/draft/basic/transports.mdx

@@ -267,6 +267,10 @@ listing its supported versions. See
 [Lifecycle: Protocol Version Negotiation][lifecycle-version]
 for the negotiation flow.
 
+If the server does not implement the requested RPC method, it **MUST** respond
+with `404 Not Found` and a JSON-RPC error with code `-32601`
+(`Method not found`).
+
 For backward compatibility, if the server does _not_ receive an
 `MCP-Protocol-Version` header and has no other way to identify the version,
 the server **SHOULD** assume protocol version `2025-03-26`.

docs/specification/draft/server/discover.mdx

@@ -82,9 +82,3 @@ is useful in two scenarios:
 
 See [Protocol Version Negotiation](/specification/draft/basic/lifecycle#protocol-version-negotiation)
 for the full version-selection flow.
-
-## Unsupported RPCs
-
-If a client calls an RPC the server does not implement, the server **MUST**
-return a `Method not found` JSON-RPC error (`-32601`). On HTTP the response
-status **MUST** be `404 Not Found`.