Update limits.mdx (#24747)

Add note on how fill rates work for our limits.

Author: kathayl

src/content/docs/browser-rendering/platform/limits.mdx

@@ -21,7 +21,9 @@ To increase this limit, you will need to [upgrade to a Workers Paid plan](/worke
 | Concurrent browsers per account (Workers Bindings only)        | 3 per account   |
 | New browser instances per minute (Workers Bindings only)      | 3 per minute    |
 | Browser timeout                        | 60 seconds |
-| Total requests per min (REST API only) | 6 per minute    |
+| Total requests per min (REST API only) | 6 per minute  [^1]  |
+
+[^1]: Rate limits are enforced with a fixed **per-second fill rate**. For example, a limit of 60 requests per minute translates to **1 request per second**. This means you cannot send all 60 requests at once; the API expects them to be spread evenly over the minute. If your account has a custom higher limit, it will also be enforced as a per-second rate.
 
 :::note[Note on browser timeout]
 By default, a browser will time out if it does not get any [devtools](https://chromedevtools.github.io/devtools-protocol/) command for 60 seconds, freeing one instance. Users can optionally increase this by using the [`keep_alive` option](/browser-rendering/platform/puppeteer/#keep-alive). `browser.close()` releases the browser instance.
@@ -35,12 +37,13 @@ The limits for Browser Rendering will continue to be raised over time. In the me
 
 | Feature                                | Limit               |
 | -------------------------------------- | ------------------- |
-| Concurrent browsers per account (Workers Bindings only)        | 10 per account [^1] |
-| New browser instances per minute (Workers Bindings only)       | 10 per minute [^1]  |
+| Concurrent browsers per account (Workers Bindings only)        | 10 per account [^2] |
+| New browser instances per minute (Workers Bindings only)       | 10 per minute [^2]  |
 | Browser timeout                        | 60 seconds |
-| Total requests per min (REST API only) | 60 per minute [^1]       |
+| Total requests per min (REST API only) | 60 per minute [^1] [^2]       |
 
-[^1]: Contact our team to request increases to this limit.
+[^1]: Rate limits are enforced with a fixed **per-second fill rate**. For example, a limit of 60 requests per minute translates to **1 request per second**. This means you cannot send all 60 requests at once; the API expects them to be spread evenly over the minute. If your account has a custom higher limit, it will also be enforced as a per-second rate.
+[^2]: Contact our team to request increases to this limit.
 
 :::note[Note on browser timeout]
 By default, a browser will time out if it does not get any [devtools](https://chromedevtools.github.io/devtools-protocol/) command for 60 seconds, freeing one instance. Users can optionally increase this by using the [`keep_alive` option](/browser-rendering/platform/puppeteer/#keep-alive). `browser.close()` releases the browser instance.