Document that the ctrl_id argument must be nonzero

Document that the ctrl_id argument of aom_codec_control() must be
nonzero. This is enforced at the beginning of aom_codec_control():

  // Control ID must be non-zero.
  if (!ctrl_id) {
    ctx->err = AOM_CODEC_INVALID_PARAM;
    return AOM_CODEC_INVALID_PARAM;
  }

Bug: aomedia:3223
Change-Id: Iec9b0fd19c02fe2013682be1f0e5834e6be488d2
(cherry picked from commit f03b45ec92ca94394f157e81cab496c6e85ea3b6)

aom/aom_codec.h

diff --git a/aom/aom_codec.h b/aom/aom_codec.h
index a2a9efa..49d48cf 100644
--- a/aom/aom_codec.h
+++ b/aom/aom_codec.h
@@ -479,14 +479,15 @@
  * ctx->err will be set to the same value as the return value.
  *
  * \param[in]     ctx              Pointer to this instance's context
- * \param[in]     ctrl_id          Algorithm specific control identifier
+ * \param[in]     ctrl_id          Algorithm specific control identifier.
+ *                                 Must be nonzero.
  *
  * \retval #AOM_CODEC_OK
  *     The control request was processed.
  * \retval #AOM_CODEC_ERROR
  *     The control request was not processed.
  * \retval #AOM_CODEC_INVALID_PARAM
- *     The data was not valid.
+ *     The control ID was zero, or the data was not valid.
  */
 aom_codec_err_t aom_codec_control(aom_codec_ctx_t *ctx, int ctrl_id, ...);