usingcurl/transfers/request-rate.md: document the 8.10 --rate extension

Closes #507

usingcurl/transfers/request-rate.md

@@ -42,3 +42,17 @@ Make curl download 10 images but doing it no faster than 3 transfers per hour:
 Make curl download 200 images but not faster than 14 transfers per minute:
 
     curl --rate 14/m -O https://example.com/[1-200].jpg
+
+## Number of units
+
+Starting in curl 8.10.0 this option accepts an optional *number of units*. The
+request rate is then provided as **N** / **Z** **U** (with no spaces) where
+**N** is a number, **Z** is a number of time units and **U** is a time unit.
+
+The number and the units are the same as mentioned above, but now you can also
+specify a number of time units. **N** requests per **Z** number of **N**
+units.
+
+For example, make no more than eight transfers every five seconds:
+
+    curl --rate 8/5s -O https://example.com/[1-100].jpg