document the single threaded wasm task pool (#4571)

# Objective

- The single threaded task pool is not documented
- This doesn't warn in CI as it's feature gated for wasm, but I'm tired of seeing the warnings when building in wasm

## Solution

- Document it

crates/bevy_tasks/src/single_threaded_task_pool.rs

@@ -14,18 +14,22 @@ impl TaskPoolBuilder {
         Self::default()
     }
 
+    /// No op on the single threaded task pool
     pub fn num_threads(self, _num_threads: usize) -> Self {
         self
     }
 
+    /// No op on the single threaded task pool
     pub fn stack_size(self, _stack_size: usize) -> Self {
         self
     }
 
+    /// No op on the single threaded task pool
     pub fn thread_name(self, _thread_name: String) -> Self {
         self
     }
 
+    /// Creates a new [`TaskPool`]
     pub fn build(self) -> TaskPool {
         TaskPool::new_internal()
     }
@@ -83,16 +87,15 @@ impl TaskPool {
             .collect()
     }
 
-    // Spawns a static future onto the JS event loop. For now it is returning FakeTask
-    // instance with no-op detach method. Returning real Task is possible here, but tricky:
-    // future is running on JS event loop, Task is running on async_executor::LocalExecutor
-    // so some proxy future is needed. Moreover currently we don't have long-living
-    // LocalExecutor here (above `spawn` implementation creates temporary one)
-    // But for typical use cases it seems that current implementation should be sufficient:
-    // caller can spawn long-running future writing results to some channel / event queue
-    // and simply call detach on returned Task (like AssetServer does) - spawned future
-    // can write results to some channel / event queue.
-
+    /// Spawns a static future onto the JS event loop. For now it is returning FakeTask
+    /// instance with no-op detach method. Returning real Task is possible here, but tricky:
+    /// future is running on JS event loop, Task is running on async_executor::LocalExecutor
+    /// so some proxy future is needed. Moreover currently we don't have long-living
+    /// LocalExecutor here (above `spawn` implementation creates temporary one)
+    /// But for typical use cases it seems that current implementation should be sufficient:
+    /// caller can spawn long-running future writing results to some channel / event queue
+    /// and simply call detach on returned Task (like AssetServer does) - spawned future
+    /// can write results to some channel / event queue.
     pub fn spawn<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
     where
         T: 'static,
@@ -103,6 +106,7 @@ impl TaskPool {
         FakeTask
     }
 
+    /// Spawns a static future on the JS event loop. This is exactly the same as [`TaskSpool::spawn`].
     pub fn spawn_local<T>(&self, future: impl Future<Output = T> + 'static) -> FakeTask
     where
         T: 'static,
@@ -115,9 +119,13 @@ impl TaskPool {
 pub struct FakeTask;
 
 impl FakeTask {
+    /// No op on the single threaded task pool
     pub fn detach(self) {}
 }
 
+/// A `TaskPool` scope for running one or more non-`'static` futures.
+///
+/// For more information, see [`TaskPool::scope`].
 #[derive(Debug)]
 pub struct Scope<'scope, T> {
     executor: &'scope async_executor::LocalExecutor<'scope>,
@@ -126,10 +134,22 @@ pub struct Scope<'scope, T> {
 }
 
 impl<'scope, T: Send + 'scope> Scope<'scope, T> {
+    /// Spawns a scoped future onto the thread-local executor. The scope *must* outlive
+    /// the provided future. The results of the future will be returned as a part of
+    /// [`TaskPool::scope`]'s return value.
+    ///
+    /// On the single threaded task pool, it just calls [`Scope::spawn_local`].
+    ///
+    /// For more information, see [`TaskPool::scope`].
     pub fn spawn<Fut: Future<Output = T> + 'scope + Send>(&mut self, f: Fut) {
         self.spawn_local(f);
     }
 
+    /// Spawns a scoped future onto the thread-local executor. The scope *must* outlive
+    /// the provided future. The results of the future will be returned as a part of
+    /// [`TaskPool::scope`]'s return value.
+    ///
+    /// For more information, see [`TaskPool::scope`].
     pub fn spawn_local<Fut: Future<Output = T> + 'scope>(&mut self, f: Fut) {
         let result = Arc::new(Mutex::new(None));
         self.results.push(result.clone());