pr47: update documentation

lib/bigstringaf.mli

@@ -164,11 +164,11 @@ val blit_to_bytes : t -> src_off:int -> Bytes.t -> dst_off:int -> len:int -> uni
 val memcmp        : t -> int -> t      -> int -> int -> int
 val memcmp_string : t -> int -> string -> int -> int -> int
 
-(** {4 [memchr]}
+(** {3 [memchr]}
+
+    Search for a byte using [memchr], returning [-1] if the byte is not found.
+    Performing bounds checking before the C call. *)
 
-    Search for a byte in a bigstring using [memchr]. Similar to [unsafe_memchr]
-    but this performs bounds checks.
-    *)
 val memchr : t -> int -> char -> int -> int
 
 (** {2 Memory-unsafe Operations}
@@ -274,8 +274,9 @@ val unsafe_blit_to_bytes : t -> src_off:int -> Bytes.t -> dst_off:int -> len:int
 val unsafe_memcmp        : t -> int -> t      -> int -> int -> int
 val unsafe_memcmp_string : t -> int -> string -> int -> int -> int
 
-(** {4 [memchr]}
+(** {3 [memchr]}
+
+    Search for a byte using [memchr], returning [-1] if the byte is not found.
+    It does not check bounds before the C call. *)
 
-    Search for a byte in a bigstring using [memchr].
-  *)
 val unsafe_memchr : t -> int -> char -> int -> int