Add documentation for custom rounding functions (#4030)

dev-docs/publisher-api-reference/setConfig.md

@@ -26,6 +26,7 @@ Core config:
 + [Set the page URL](#setConfig-Page-URL)
 + [Set price granularity](#setConfig-Price-Granularity)
 + [Set media type price granularity](#setConfig-MediaType-Price-Granularity)
++ [Set custom cpm rounding](#setConfig-Cpm-Rounding)
 + [Configure server-to-server header bidding](#setConfig-Server-to-Server)
 + [Configure user syncing](#setConfig-Configure-User-Syncing)
 + [Configure targeting controls](#setConfig-targetingControls)
@@ -420,6 +421,37 @@ are recognized. This was driven by the recognition that outstream often shares l
 If the mediatype is video, the price bucketing code further looks at the context (e.g. outstream) to see if there's
 a price granularity override. If it doesn't find 'video-outstream' defined, it will then look for just 'video'.
 
+<a name="setConfig-Cpm-Rounding" />
+
+#### Custom CPM Rounding
+
+Prebid defaults to rounding down all bids to the nearest increment, which may cause lower CPM ads to be selected. 
+While this can be addressed through higher [price granularity](#setConfig-Price-Granularity), Prebid also allows setting a custom rounding function. 
+This function will be used by Prebid to determine what increment a bid will round to. 
+<br/>
+<br/>
+You can set a simple rounding function:
+```javascript
+// Standard rounding
+pbjs.setConfig({'cpmRoundingFunction': Math.round});
+```
+
+Or you can round according to more complex considerations:
+
+```javascript
+// Custom rounding function
+const roundToNearestEvenIncrement = function (number) {
+  let ceiling = Math.ceil(number);
+  let ceilingIsEven = ceiling % 2 === 0;
+  if (ceilingIsEven) {
+    return ceiling;
+  } else {
+    return Math.floor(number);
+  }
+}
+pbjs.setConfig({'cpmRoundingFunction': roundToNearestEvenIncrement});
+```
+
 <a name="setConfig-Server-to-Server" />
 
 #### Server to Server