Clarify that ::Create does not enforce 1-per context. Add a test.

impeller/renderer/backend/vulkan/BUILD.gn

@@ -11,6 +11,7 @@ impeller_component("vulkan_unittests") {
     "blit_command_vk_unittests.cc",
     "context_vk_unittests.cc",
     "pass_bindings_cache_unittests.cc",
+    "resource_manager_vk_unittests.cc",
     "test/mock_vulkan.cc",
     "test/mock_vulkan.h",
   ]

impeller/renderer/backend/vulkan/resource_manager_vk.h

@@ -37,9 +37,14 @@ class ResourceManagerVK
     : public std::enable_shared_from_this<ResourceManagerVK> {
  public:
   //----------------------------------------------------------------------------
-  /// @brief      Create a shared resource manager. This creates a dedicated
-  ///             thread for resource management. Only one is created per Vulkan
-  ///             context.
+  /// @brief      Creates a shared resource manager (a dedicated thread).
+  ///
+  /// Upon creation, a thread is spawned which will collect resources as they
+  /// are reclaimed (passed to `Reclaim`). The thread will exit when the
+  /// resource manager is destroyed.
+  ///
+  /// @note       Only one |ResourceManagerVK| should be created per Vulkan
+  ///             context, but that contract is not enforced by this method.
   ///
   /// @return     A resource manager if one could be created.
   ///

impeller/renderer/backend/vulkan/resource_manager_vk_unittests.cc

@@ -0,0 +1,20 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <sys/types.h>
+#include "flutter/testing/testing.h"
+#include "impeller/renderer/backend/vulkan/resource_manager_vk.h"
+
+namespace impeller {
+namespace testing {
+
+// While expected to be a singleton per context, the class does not enforce it.
+TEST(ResourceManagerVKTest, CreateCreatesANewInstance) {
+  auto const a = ResourceManagerVK::Create();
+  auto const b = ResourceManagerVK::Create();
+  EXPECT_NE(a, b);
+}
+
+}  // namespace testing
+}  // namespace impeller