Adjust protocol.md

Author: arnaudgolfouse

protocol.md

@@ -18,23 +18,33 @@ Valid plugins need to import two functions (that will be provided by the runtime
 
   Write the arguments for the current function into the buffer pointed at by `ptr`.
 
-  Each function for the protocol receives lengths as its arguments (see [Exported functions](#exported-functions)). The capacity of the buffer pointed at by `ptr` should be at least the sum of all those lengths.
+  Each function for the protocol receives lengths as its arguments (see [User-defined functions](#user-defined-functions)). The capacity of the buffer pointed at by `ptr` should be at least the sum of all those lengths.
 
 - `(import "typst_env" "wasm_minimal_protocol_send_result_to_host" (func (param i32 i32)))`
 
   The first parameter is a pointer to a buffer (`ptr`), the second is the length of the buffer (`len`).
 
-  Reads `len` bytes pointed at by `ptr` into host memory. The memory pointed at by `ptr` can be freed immediately after this function returns.
+  Send `len` and `ptr` to host memory. The buffer must not be freed by the end of the function: it will be freed by the runtime by calling [`wasm_minimal_protocol_send_result_to_host`](#exports).
 
-  If the message should be interpreted as an error message (see [Exported functions](#exported-functions)), it should be encoded as UTF-8.
+  If the message should be interpreted as an error message (see [User-defined functions](#user-defined-functions)), it should be encoded as UTF-8.
 
-# Exported functions
+  ### Note
+
+  If [`wasm_minimal_protocol_send_result_to_host`](#exports) calls `free` (or a similar routine), be careful that the buffer does not point to static memory.
+
+# Exports
+
+Valid plugins need to export a function named `wasm_minimal_protocol_send_result_to_host`, that has signature `func (param i32 i32)`.
+
+This function will be used by the runtime to free the block of memory returned by a [user-defined](#user-defined-functions) function.
+
+# User-defined functions
 
 To conform to the protocol, an exported function should:
 
-- Take `n` arguments `a₁`, `a₂`, ..., `aₙ` of type `u32` (interpreted as lengths, so `usize/size_t` may be preferable), and return one `i32`.
+- Take `n` arguments `a₁`, `a₂`, ..., `aₙ` of type `u32` (interpreted as lengths, so `usize/size_t` may be preferable), and return one `i32`. We will call the return `return_code`.
 - The function should first allocate a buffer `buf` of length `a₁ + a₂ + ⋯ + aₙ`, and call `wasm_minimal_protocol_write_args_to_buffer(buf.ptr)`.
 - The `a₁` first bytes of the buffer constitute the first argument, the `a₂` next bytes the second argument, and so on.
 - Before returning, the function should call `wasm_minimal_protocol_send_result_to_host` to send its result back to the host.
-- To signal success, the function should return `0`.
-- To signal an error, the function should return `1`. The written buffer is then interpreted as an error message.
+- To signal success, `return_code` must be `0`.
+- To signal an error, `return_code` must be `1`. The sent buffer is then interpreted as an error message, and must be encoded as UTF-8.