Changes description on when to use signed_video_get_sei(...)

The proper way is to call it BEFORE adding NAL Units for signing. Then
the user can peek the next NAL Unit to get the SEI if and only if it
can be prepended the peeked NAL Unit and at the same time follow the
H.26x standard when using AU format.

This commit only updates the description. Tests are updated
separately.

lib/src/includes/signed_video_sign.h

@@ -229,17 +229,17 @@ signed_video_get_nalu_to_prepend(signed_video_t *self,
 /**
  * @brief Gets generated SEIs to add to the stream
  *
- * This function should always be called after a successful
- * signed_video_add_nalu_for_signing_with_timestamp(...). Otherwise, the functionality of Signed
- * Video is jeopardized, since vital SEI-NALUs may not be added to the video stream. The
- * signed_video_add_nalu_for_signing_with_timestamp(...) returns SV_NOT_SUPPORTED if there are
- * available NALUs to prepend. The user should then pull these before continuing; See return values
- * in signed_video_add_nalu_for_signing_with_timestamp(...).
- *
- * These SEI-NALUs are generated by the Signed Video library and are complete NALUs + 4 start code
- * bytes. Hence, the user can simply pull and prepend existing H26x NALUs. Pull NALUs to prepend
- * from signed_video_t one by one until no more generated SEIs exists, that is, when |sei_size|
- * is zero.
+ * This function should be called before
+ * signed_video_add_nalu_for_signing_with_timestamp(...). The user has an option to
+ * provide this function with a |peek_nalu|, which is the same NAL Unit that is to be
+ * added for signing. Even though, the user does not have to call this function before
+ * adding NAL Units for signing, it is highly recommended to avoid missing or delaying
+ * SEIs.
+ *
+ * These SEIs are generated by the Signed Video library and are complete NAL Units + 4
+ * start code bytes. Hence, the user can simply pull and prepend existing H26x NAL Units.
+ * Pull NAL Units to prepend from signed_video_t one by one until no more generated SEIs
+ * exists, that is, when |sei_size| is zero.
  *
  * NOTE: The user is responsible for allocating memory for the |sei|.
  *
@@ -254,27 +254,24 @@ signed_video_get_nalu_to_prepend(signed_video_t *self,
  *     // Handle error
  *   }
  *   SignedVideoReturnCode status;
+ *   size_t sei_size = 0;
+ *   // The first call of the function is for getting the |sei_size|.
+ *   status = signed_video_get_sei(sv, NULL, &sei_size, NULL, 0, NULL);
+ *   // The second call is to get the sei.
+ *   while (status == SV_OK && sei_size > 0) {
+ *       uint8_t *sei = malloc(sei_size);
+ *       status = signed_video_get_sei(sv, sei, &sei_size, NULL, 0, NULL);
+ *       if (status != SV_OK) {
+ *        // True error. Handle it properly.
+ *       }
+ *       // Add the SEI to the stream according to the standard.
+ *       // The user is responsible for freeing |sei|.
+ *       // Check for more SEIs.
+ *       status = signed_video_get_sei(sv, NULL, &sei_size, NULL, 0, NULL);
+ *   }
  *   status = signed_video_add_nalu_for_signing_with_timestamp(sv, nalu, nalu_size, NULL);
  *   if (status != SV_OK) {
  *     // Handle error
- *   } else {
- *     size_t sei_size = 0;
- *     status = signed_video_get_sei(sv, NULL, &sei_size, NULL, 0, NULL);
- *     // The first call of the function is for getting the |sei_size|.
- *     // The second call is to get the sei.
- *     while (status == SV_OK && sei_size > 0) {
- *         uint8_t *sei = malloc(sei_size);
- *         status = signed_video_get_sei(sv, sei, &sei_size, NULL, 0, NULL);
- *         if (status != SV_OK) {
- *          // True error. Handle it properly.
- *         }
- *         // The user is responsible for freeing |sei|.
- *         status = signed_video_get_sei(sv, NULL, &sei_size, NULL, 0, NULL);
- *     }
- *     // Handle return code
- *     if (status != SV_OK) {
- *       // True error. Handle it properly.
- *     }
  *   }
  *
  * @param self Pointer to the signed_video_t object in use.
@@ -288,9 +285,9 @@ signed_video_get_nalu_to_prepend(signed_video_t *self,
  * @param peek_nalu_size The size of the peek NAL Unit.
  * @param num_pending_seis Pointer to where the number of pending SEIs is written.
  *
- * @returns SV_OK            - NALU was copied successfully,
- *          SV_NOT_SUPPORTED - no available data, the action is not supported,
- *          otherwise        - an error code.
+ * @return SV_OK            - NALU was copied successfully,
+ *         SV_NOT_SUPPORTED - no available data, the action is not supported,
+ *         otherwise        - an error code.
  */
 SignedVideoReturnCode
 signed_video_get_sei(signed_video_t *self,