GPU: Clarify some presentation concerns in docs (#15707)

include/SDL3/SDL_gpu.h

@@ -4232,10 +4232,12 @@ extern SDL_DECLSPEC SDL_GPUTextureFormat SDLCALL SDL_GetGPUSwapchainTextureForma
  * submitted. The swapchain texture should only be referenced by the command
  * buffer used to acquire it.
  *
- * This function will fill the swapchain texture handle with NULL if too many
- * frames are in flight. This is not an error. This NULL pointer should not be
+ * If too many frames are in flight, this function will fill the swapchain texture handle
+ * with NULL and return true. This is not an error. This NULL pointer should not be
  * passed back into SDL. Instead, it should be considered as an indication to
- * wait until the swapchain is available.
+ * wait.
+ *
+ * In VSYNC present mode (which is the default) this function may block on vblank.
  *
  * If you use this function, it is possible to create a situation where many
  * command buffers are allocated while the rendering context waits for the GPU
@@ -4280,7 +4282,7 @@ extern SDL_DECLSPEC bool SDLCALL SDL_AcquireGPUSwapchainTexture(
     Uint32 *swapchain_texture_height);
 
 /**
- * Blocks the thread until a swapchain texture is available to be acquired.
+ * Blocks the thread until all presenting command buffers are finished executing.
  *
  * \param device a GPU context.
  * \param window a window that has been claimed.