docs: simplify RPC guide error and MCP wording

Remove the "if X it throws DFxxxx" phrasings from the RPC guide, drop the redundant zod tip and args/returns warning callouts, and fold the MCP jsonSerializable requirement into a single line under the Agent exposure example.

Author: antfubot

docs/content/1.guide/2.rpc.md

@@ -56,13 +56,7 @@ Scope with your devframe id, then a kebab-case action: `my-devframe:get-modules`
 
 ### Handler arguments
 
-Handlers accept any serializable arguments. Declared `args` schemas validate each at the boundary; extra object fields still reach the handler.
-
-> [!TIP]
-> If your app already pulls in **zod**, use it for your RPC schemas to reuse a dependency you already ship.
-
-> [!WARNING]
-> Declared `args`/`returns` schemas are enforced at runtime; failing arguments or return values are rejected with `DF0043` / `DF0044`.
+Handlers accept any serializable arguments. Declared `args` and `returns` schemas are enforced at runtime; extra object fields still reach the handler.
 
 ### Setup vs handler
 
@@ -198,15 +192,7 @@ The WS transport picks one of two encoders per function:
 | `false` (default) | `structured-clone-es` | `s:` | `Map`, `Set`, `Date`, `BigInt`, cycles, class instances |
 | `true` (opt-in) | strict `JSON.stringify` | _(unprefixed)_ | JSON-only |
 
-When every function is JSON-flagged, the wire stays plain JSON.
-
-### Discovering shape errors during dev
-
-When a `jsonSerializable: true` handler returns a value JSON cannot round-trip (`Map`, `Date`, …), the strict serializer throws [`DF0020`](/errors/DF0020).
-
-### MCP requires JSON
-
-`agent: {...}` requires `jsonSerializable: true`; one without the other throws [`DF0019`](/errors/DF0019).
+When every function is JSON-flagged, the wire stays plain JSON. A `jsonSerializable: true` handler must return JSON-only values — `Map`, `Date`, and friends won't round-trip.
 
 ## Agent exposure
 
@@ -230,6 +216,8 @@ defineRpcFunction({
 })
 ```
 
+Exposing a function over MCP requires `jsonSerializable: true`.
+
 ## What's next
 
 - [Shared State](/guide/shared-state) — state synced across clients