MINOR: ncbuf: complete doc for ncb_advance()

ncb_advance() operation may reject the operation if a too small gap is
formed in buffer front. This must be documented to avoid an issue with
it.

This should be backported up to 2.6.

(cherry picked from commit 7f0295f08a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

src/ncbuf.c

@@ -625,7 +625,9 @@ enum ncb_ret ncb_add(struct ncbuf *buf, ncb_sz_t off,
  * will be lost while some space will be formed at the end to be able to insert
  * new data.
  *
- * Returns NCB_RET_OK on success.
+ * Returns NCB_RET_OK on success. It may return NCB_RET_GAP_SIZE if operation
+ * is rejected due to the formation of a too small gap in front. If advance is
+ * done only inside a data block it is guaranteed to succeed.
  */
 enum ncb_ret ncb_advance(struct ncbuf *buf, ncb_sz_t adv)
 {