[Impeller] Documentation for SurfaceContextVK (flutter/engine#49882)

See #44171

engine/src/flutter/impeller/renderer/backend/vulkan/surface_context_vk.h

@@ -17,6 +17,16 @@ class ContextVK;
 class Surface;
 class SwapchainVK;
 
+/// For Vulkan, there is both a ContextVK that implements Context and a
+/// SurfaceContextVK that also implements Context and takes a ContextVK as its
+/// parent. There is a one to many relationship between ContextVK and
+/// SurfaceContextVK.
+///
+/// Most operations in this class are delegated to the parent ContextVK.
+/// This class specifically manages swapchains and creation of VkSurfaces on
+/// Android. By maintaining the swapchain this way, it is possible to have
+/// multiple surfaces sharing the same ContextVK without stepping on each
+/// other's swapchains.
 class SurfaceContextVK : public Context,
                          public BackendCast<SurfaceContextVK, Context> {
  public: