Clarify rate limiter expressions in addsvc example

[marselester]

Rate limiting examples might be a bit confusing when rate.Every method is used #698.
Naturally one would expect to see a throttling demo as a number of requests per unit of time

// 100 requests per second with burst of 100 requests.
rate.NewLimiter(rate.Limit(100), 100)

// 5 requests per minute with burst of 5 requests.
rate.NewLimiter(rate.Limit(5/time.Minute.Seconds()), 5)

instead of defining an interval between requests.

// 1 request per second with burst of 100 requests.
rate.NewLimiter(rate.Every(time.Second), 100)

// 5 requests per minute with burst of 5 requests.
rate.NewLimiter(rate.Every(time.Minute/time.Duration(5)), 5)

[peterbourgon]

The different ways to express rate limits and burst capacity are properties of the rate limiting package, and it's expected that users will read those docs to learn more. It's not clear to me that your changes are strictly or even generally easier to understand, so I'm -1 on a wholesale change, but I'd welcome a change to one example, and comments (like you have in the comments) on both examples explaining what the expressions mean.

[marselester]

I added comments to addsvc example since it has two rate limiters. It should be easier to spot the difference when comparing expressions side by side.