I found the following in ./en/MonoTouch.WebKit/WKContentRuleListStore.xml:
<para class="improve-task-t-return-type-description">A task that represents the asynchronous CompileContentRuleList operation. The value of the TResult parameter is of type MonoTouch.WebKit.WKContentRuleList,MonoTouch.Foundation.NSError.</para>
The specified "type" is the argument list for the completion handler in the non-Async version of the method. There is not a simple rule that generates the Task return type from the completion handler. If these descriptions are necessary, please use the actual Task type from the Async method (System.Threading.Tasks.Task<MonoTouch.WebKit.WKContentRuleList>, in this case) to fill out this field, instead.
While a text search will yield the method, I found it in CompileContentRuleListAsync()
This was fix in a PR related to a diff bug -> https://github.com/xamarin/maccore/pull/561