avutil/buffer: avutil/buffer: add a mention that some arguments from av_buffer_pool_init2() may be NULL

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/buffer.h

@@ -254,12 +254,13 @@ AVBufferPool *av_buffer_pool_init(int size, AVBufferRef* (*alloc)(int size));
  * @param size size of each buffer in this pool
  * @param opaque arbitrary user data used by the allocator
  * @param alloc a function that will be used to allocate new buffers when the
- *              pool is empty.
+ *              pool is empty. May be NULL, then the default allocator will be
+ *              used (av_buffer_alloc()).
  * @param pool_free a function that will be called immediately before the pool
  *                  is freed. I.e. after av_buffer_pool_uninit() is called
  *                  by the caller and all the frames are returned to the pool
  *                  and freed. It is intended to uninitialize the user opaque
- *                  data.
+ *                  data. May be NULL.
  * @return newly created buffer pool on success, NULL on error.
  */
 AVBufferPool *av_buffer_pool_init2(int size, void *opaque,