lavc: clarify get_buffer() documentation

This is needed for the AAC decoder, which may need to call get_buffer()
more than once if the channel configuration changes.

libavcodec/avcodec.h

@@ -2218,7 +2218,12 @@ typedef struct AVCodecContext {
      *
      * Decoders cannot use the buffer after returning from
      * avcodec_decode_audio4(), so they will not call release_buffer(), as it
-     * is assumed to be released immediately upon return.
+     * is assumed to be released immediately upon return. In some rare cases,
+     * a decoder may need to call get_buffer() more than once in a single
+     * call to avcodec_decode_audio4(). In that case, when get_buffer() is
+     * called again after it has already been called once, the previously
+     * acquired buffer is assumed to be released at that time and may not be
+     * reused by the decoder.
      *
      * As a convenience, av_samples_get_buffer_size() and
      * av_samples_fill_arrays() in libavutil may be used by custom get_buffer()