Enable lock for Vector to protect wasm-c-api read/write/extend operations (#1010)

doc/wasm_c_api.md

@@ -3,24 +3,42 @@
 All samples come from the commit 340fd9528cc3b26d22fe30ee1628c8c3f2b8c53b
 of [wasm-c-api](https://github.com/WebAssembly/wasm-c-api).
 
-Developer can learn these *APIs* from
+Developer can learn these _APIs_ from
 [wasm.h](https://github.com/WebAssembly/wasm-c-api/blob/master/include/wasm.h).
 
 And here are [examples](https://github.com/WebAssembly/wasm-c-api/tree/master/example) which
 are helpful.
 
+## FYI
+
+- The thread model of _wasm_c_api_ is
+
+  - An `wasm_engine_t` instance may only be created once per process
+  - Every `wasm_store_t` and its objects may only be accessed in a single thread
+
+- `wasm_engine_new`, `wasm_engine_new_with_config`, `wasm_engine_new_with_args`,
+  `wasm_engine_delete`should be called in a thread-safe environment. Such
+  behaviors are not recommended, and please make sure an appropriate calling
+  sequence if it has to be.
+
+  - call `wasm_engine_new` and `wasm_engine_delete` in different threads
+  - call `wasm_engine_new` or `wasm_engine_delete` multiple times in
+    different threads
+
+## unspported list
+
 Currently WAMR supports most of the APIs, the unsupported APIs are listed as below:
 
 - References
 
-``` c
+```c
 WASM_API_EXTERN own wasm_shared_##name##_t* wasm_##name##_share(const wasm_##name##_t*);
 WASM_API_EXTERN own wasm_##name##_t* wasm_##name##_obtain(wasm_store_t*, const wasm_shared_##name##_t*);
 ```
 
 - Several Module APIs
 
-``` c
+```c
 WASM_API_EXTERN void wasm_module_serialize(const wasm_module_t*, own wasm_byte_vec_t* out);
 WASM_API_EXTERN own wasm_module_t* wasm_module_deserialize(wasm_store_t*, const wasm_byte_vec_t*);
 ```
@@ -30,12 +48,12 @@ by host-side function callings.
 
 - Table Grow APIs
 
-``` c
+```c
 WASM_API_EXTERN bool wasm_table_grow(wasm_table_t*, wasm_table_size_t delta, wasm_ref_t* init);
 ```
 
 - Memory Grow APIs
 
-``` c
+```c
 WASM_API_EXTERN bool wasm_memory_grow(wasm_memory_t*, wasm_memory_pages_t delta);
 ```