avcodec/avcodec: Don't suggest calling deprecated avcodec_register_all()

Despite avcodec_register and avcodec_register_all being deprecated,
their documentation still said that one of them has to be called
before doing anything else. Clarify this confusing situation.

Furthermore, don't use avcodec_register_all in sample code for
a non-deprecated function.

Reviewed-by: mypopy@gmail.com <mypopy@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/avcodec.h

@@ -2708,25 +2708,13 @@ const char *avcodec_license(void);
 
 #if FF_API_NEXT
 /**
- * Register the codec codec and initialize libavcodec.
- *
- * @warning either this function or avcodec_register_all() must be called
- * before any other libavcodec functions.
- *
- * @see avcodec_register_all()
+ * @deprecated Calling this function is unnecessary.
  */
 attribute_deprecated
 void avcodec_register(AVCodec *codec);
 
 /**
- * Register all the codecs, parsers and bitstream filters which were enabled at
- * configuration time. If you do not call this function you can select exactly
- * which formats you want to support, by using the individual registration
- * functions.
- *
- * @see avcodec_register
- * @see av_register_codec_parser
- * @see av_register_bitstream_filter
+ * @deprecated Calling this function is unnecessary.
  */
 attribute_deprecated
 void avcodec_register_all(void);
@@ -2843,7 +2831,6 @@ int avcodec_parameters_to_context(AVCodecContext *codec,
  * @ref avcodec_receive_frame()).
  *
  * @code
- * avcodec_register_all();
  * av_dict_set(&opts, "b", "2.5M", 0);
  * codec = avcodec_find_decoder(AV_CODEC_ID_H264);
  * if (!codec)