lavc: fix and extend AVCodecContext.hwaccel_context doxy

Mention:
- that it is legacy and optional (every hwaccel that uses it can also
  work with hwcontext, though some optional information can only be
  signalled throught hwaccel_context)
- that it can be used for encoders (only qsvenc currently)
- ownership and lifetime

libavcodec/avcodec.h

@@ -1388,13 +1388,26 @@ typedef struct AVCodecContext {
     const struct AVHWAccel *hwaccel;
 
     /**
-     * Hardware accelerator context.
-     * For some hardware accelerators, a global context needs to be
-     * provided by the user. In that case, this holds display-dependent
-     * data FFmpeg cannot instantiate itself. Please refer to the
-     * FFmpeg HW accelerator documentation to know how to fill this.
-     * - encoding: unused
-     * - decoding: Set by user
+     * Legacy hardware accelerator context.
+     *
+     * For some hardware acceleration methods, the caller may use this field to
+     * signal hwaccel-specific data to the codec. The struct pointed to by this
+     * pointer is hwaccel-dependent and defined in the respective header. Please
+     * refer to the FFmpeg HW accelerator documentation to know how to fill
+     * this.
+     *
+     * In most cases this field is optional - the necessary information may also
+     * be provided to libavcodec through @ref hw_frames_ctx or @ref
+     * hw_device_ctx (see avcodec_get_hw_config()). However, in some cases it
+     * may be the only method of signalling some (optional) information.
+     *
+     * The struct and its contents are owned by the caller.
+     *
+     * - encoding: May be set by the caller before avcodec_open2(). Must remain
+     *             valid until avcodec_free_context().
+     * - decoding: May be set by the caller in the get_format() callback.
+     *             Must remain valid until the next get_format() call,
+     *             or avcodec_free_context() (whichever comes first).
      */
     void *hwaccel_context;