rate-limit: Clarify token bucket creation docs

Creating a token bucket fails if the rate limit supplied is 0 or
negative - this can lead to unexpected and undesirable behaviour, such as
division by 0 or negative token counts.

Signed-off-by: Christian Pardillo Laursen <[email protected]>

Author: cplaursen

ocaml/libs/rate-limit/token_bucket.mli

@@ -36,6 +36,7 @@ type t
 
 val create : burst_size:float -> fill_rate:float -> t option
 (** Create token bucket with given parameters.
+    Returns None if the fill rate is 0 or negative.
     @param burst_size Maximum number of tokens that can fit in the bucket
     @param fill_rate Number of tokens added to the bucket per second
     *)
@@ -70,14 +71,15 @@ val delay_then_consume : t -> float -> unit
 val create_with_timestamp :
   Mtime.span -> burst_size:float -> fill_rate:float -> t option
 (** Create token bucket with given parameters and supplied inital timestamp
+    Returns None if the fill_rate is 0 or negative.
     @param timestamp Initial timestamp
     @param burst_size Maximum number of tokens that can fit in the bucket
     @param fill_rate Number of tokens added to the bucket per second
     *)
 
 val peek_with_timestamp : Mtime.span -> t -> float
 (** Retrieve token amount in token bucket at given timestamp.
-      Undefined behaviour when [timestamp] <= [tb.timestamp]
+    Undefined behaviour when [timestamp] <= [tb.timestamp]
      @param timestamp Current time
      @param tb Token bucket
      @return Amount of tokens in the token bucket