Document withMainSerialExecutor caveats (#3)

* Add warning to README

* wip

Author: stephencelis

README.md

@@ -156,8 +156,16 @@ purpose, it has not landed in Swift.
 
 Some asynchronous code is [notoriously difficult][reliably-testing-swift-concurrency] to test in
 Swift due to how suspension points are processed by the runtime. The library comes with a static
-function, `withMainSerialExecutor`, that runs all tasks spawned in an operation serially and
-deterministically. This function can be used to make asynchronous tests faster and less flakey.
+function, `withMainSerialExecutor`, that attempts to run all tasks spawned in an operation serially
+and deterministically. This function can be used to make asynchronous tests faster and less flakey.
+
+> **Warning**: This API is only intended to be used from tests to make them more reliable. Please do
+> not use it from application code.
+>
+> We say that it "_attempts_ to run all tasks spawned in an operation serially and
+> deterministically" because under the hood it relies on a global, mutable variable in the Swift
+> runtime to do its job, and there are no scoping _guarantees_ should this mutable variable change
+> during the operation.
 
 For example, consider the following seemingly simple model that makes a network request and manages
 so `isLoading` state while the request is inflight:

Sources/ConcurrencyExtras/MainSerialExecutor.swift

@@ -5,16 +5,24 @@
   ///
   /// Some asynchronous code is [notoriously
   /// difficult](https://forums.swift.org/t/reliably-testing-code-that-adopts-swift-concurrency/57304)
-  /// to test in Swift due to how suspension points are processed by the runtime. This function runs
-  /// all tasks spawned in the given operation serially and deterministically. It makes asynchronous
-  /// tests faster and less flakey.
+  /// to test in Swift due to how suspension points are processed by the runtime. This function
+  /// attempts to run all tasks spawned in the given operation serially and deterministically. It
+  /// makes asynchronous tests faster and less flakey.
   ///
   /// ```swift
   /// await withMainSerialExecutor {
   ///   // Everything performed in this scope is performed serially...
   /// }
   /// ```
   ///
+  /// > Warning: This API is only intended to be used from tests to make them more reliable. Please do
+  /// > not use it from application code.
+  /// >
+  /// > We say that it "_attempts_ to run all tasks spawned in an operation serially and
+  /// > deterministically" because under the hood it relies on a global, mutable variable in the Swift
+  /// > runtime to do its job, and there are no scoping _guarantees_ should this mutable variable change
+  /// > during the operation.
+  ///
   /// - Parameter operation: An operation to be performed on the main serial executor.
   @MainActor
   public func withMainSerialExecutor(