avcodec/avcodec: Warn about data returned from get_buffer*()

Text based on suggestion by: epirat07@gmail.com
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/APIchanges

@@ -2,6 +2,12 @@ The last version increases of all libraries were on 2024-03-07
 
 API changes, most recent first:
 
+2024-08-xx - xxxxxxxxx - lavc 61.11.100- avcodec.h
+  Clarify the documentation for get_buffer*() functions, making it
+  clear that the memory returned by them should not contain sensitive
+  information. This is not a change in the API, it is how it already worked
+  before.
+
 2024-08-10 - xxxxxxxxx - lavu 59.34.100 - hwcontext_vulkan.h
   Add qf and nb_qf to AVVulkanDeviceContext.
   Deprecate queue_family_index, nb_graphics_queues,

libavcodec/avcodec.h

@@ -1175,6 +1175,10 @@ typedef struct AVCodecContext {
      *   this callback and filled with the extra buffers if there are more
      *   buffers than buf[] can hold. extended_buf will be freed in
      *   av_frame_unref().
+     *   Decoders will generally initialize the whole buffer before it is output
+     *   but it can in rare error conditions happen that uninitialized data is passed
+     *   through. \important The buffers returned by get_buffer* should thus not contain sensitive
+     *   data.
      *
      * If AV_CODEC_CAP_DR1 is not set then get_buffer2() must call
      * avcodec_default_get_buffer2() instead of providing buffers allocated by