Document -1 extra return code in kh_put().

Author: sciascid

khash.h

@@ -455,7 +455,8 @@ static kh_inline khint_t __ac_Wang_hash(khint_t key)
   @param  name  Name of the hash table [symbol]
   @param  h     Pointer to the hash table [khash_t(name)*]
   @param  k     Key [type of keys]
-  @param  r     Extra return code: 0 if the key is present in the hash table;
+  @param  r     Extra return code: -1 if the operation failed;
+                0 if the key is present in the hash table;
                 1 if the bucket is empty (never used); 2 if the element in
 				the bucket has been deleted [int*]
   @return       Iterator to the inserted element [khint_t]