avutil/hwcontext: Clarify documentation of AVHWFramesContext.hwctx

Correct the names of the format-specific headers (not hwframe_*.h)
and clarify that the user shall ignore this field if there is no
public context associated with it.
In particular, this allows to use this field for the private context
alone if there is no public context. This can't break conforming
API users, because they always have to live with the possibility
that a new public context has been introduced.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/hwcontext.h

@@ -153,9 +153,12 @@ typedef struct AVHWFramesContext {
      * The format-specific data, allocated and freed automatically along with
      * this context.
      *
-     * Should be cast by the user to the format-specific context defined in the
-     * corresponding header (hwframe_*.h) and filled as described in the
-     * documentation before calling av_hwframe_ctx_init().
+     * The user shall ignore this field if the corresponding format-specific
+     * header (hwcontext_*.h) does not define a context to be used as
+     * AVHWFramesContext.hwctx.
+     *
+     * Otherwise, it should be cast by the user to said context and filled
+     * as described in the documentation before calling av_hwframe_ctx_init().
      *
      * After any frames using this context are created, the contents of this
      * struct should not be modified by the caller.