README-migration: Note that render target support is not universally available.

Fixes #15474.

docs/README-migration.md

@@ -1447,7 +1447,7 @@ The following functions have been removed:
 * SDL_GetTextureUserData() - use SDL_GetTextureProperties() instead
 * SDL_RenderGetIntegerScale()
 * SDL_RenderSetIntegerScale() - this is now explicit with SDL_LOGICAL_PRESENTATION_INTEGER_SCALE
-* SDL_RenderTargetSupported() - render targets are always supported
+* SDL_RenderTargetSupported() - render targets are usually supported; just create a texture with SDL_TEXTUREACCESS_TARGET and see if it fails.
 * SDL_SetTextureUserData() - use SDL_GetTextureProperties() instead
 
 The following enums have been renamed:
@@ -1461,7 +1461,7 @@ The following symbols have been removed:
 * SDL_RENDERER_ACCELERATED - all renderers except `SDL_SOFTWARE_RENDERER` are accelerated
 * SDL_RENDERER_PRESENTVSYNC - replaced with SDL_PROP_RENDERER_CREATE_PRESENT_VSYNC_NUMBER during renderer creation and SDL_PROP_RENDERER_VSYNC_NUMBER after renderer creation
 * SDL_RENDERER_SOFTWARE - you can check whether the name of the renderer is `SDL_SOFTWARE_RENDERER`
-* SDL_RENDERER_TARGETTEXTURE - all renderers support target texture functionality
+* SDL_RENDERER_TARGETTEXTURE - most renderers support target texture functionality; just create a texture with SDL_TEXTUREACCESS_TARGET and see if it fails.
 * SDL_ScaleModeBest - use SDL_SCALEMODE_LINEAR instead
 
 ## SDL_rwops.h