embed_wamr.md: Improvements about threads (#2420)

- Some more clarifications
- Mention wasi-threads
- Fix a confusing indentation

doc/embed_wamr_spawn_api.md

@@ -0,0 +1,38 @@
+# Thread related embedder API
+
+This document explains `wasm_runtime_spawn_exec_env` and
+`wasm_runtime_spawn_thread`.
+[Here](../samples/spawn-thread) is a sample to show how to use these APIs.
+
+  * spawn exec_env
+
+    `spawn exec_env` API creates a new `exec_env` based on the original `exec_env`. You can use it in other threads. It's up to the embedder how to manage host threads to run the new `exec_env`.
+
+    ```C
+    new_exec_env = wasm_runtime_spawn_exec_env(exec_env);
+
+      /* Then you can use new_exec_env in your new thread */
+      module_inst = wasm_runtime_get_module_inst(new_exec_env);
+      func_inst = wasm_runtime_lookup_function(module_inst, ...);
+      wasm_runtime_call_wasm(new_exec_env, func_inst, ...);
+
+    /* you need to use this API to manually destroy the spawned exec_env */
+    wasm_runtime_destroy_spawned_exec_env(new_exec_env);
+    ```
+
+  * spawn thread
+
+    Alternatively, you can use `spawn thread` API to avoid managing the extra exec_env and the corresponding host thread manually:
+
+    ```C
+    wasm_thread_t wasm_tid;
+    void *wamr_thread_cb(wasm_exec_env_t exec_env, void *arg)
+    {
+      module_inst = wasm_runtime_get_module_inst(exec_env);
+      func_inst = wasm_runtime_lookup_function(module_inst, ...);
+      wasm_runtime_call_wasm(exec_env, func_inst, ...);
+    }
+    wasm_runtime_spawn_thread(exec_env, &wasm_tid, wamr_thread_cb, NULL);
+    /* Use wasm_runtime_join_thread to join the spawned thread */
+    wasm_runtime_join_thread(wasm_tid, NULL);
+    ```