docs: Fix description of the leaky bucket algorithm in throttle.txt

Commit 0bab0ebb was supposed to fix
a mistake in the description of the leaky bucket algorithm, but the
version that finally landed after the review process was incorrect.

This patch solves that problem and hopefully clarifies the description
a bit better.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

docs/throttle.txt

@@ -235,7 +235,10 @@ consider the following values:
   - Water leaks from the bucket at a rate of 100 IOPS.
   - Water can be added to the bucket at a rate of 2000 IOPS.
   - The size of the bucket is 2000 x 60 = 120000
-  - If 'iops-total-max' is unset then the bucket size is 100 x 60.
+  - If 'iops-total-max-length' is unset then it defaults to 1 and the
+    size of the bucket is 2000.
+  - If 'iops-total-max' is unset then 'iops-total-max-length' must be
+    unset as well. In this case the bucket size is 100.
 
 The bucket is initially empty, therefore water can be added until it's
 full at a rate of 2000 IOPS (the burst rate). Once the bucket is full