rework pytest MockIRCServer kwarg section

_upgrading/sopel-8.md

@@ -395,13 +395,17 @@ raise a `KeyError`.
 
 ### Testing tool changes
 
-* Convenience methods of the mock IRC server available through Sopel's `pytest`
-  plugin implement keyword-only arguments now. This applies to:
-  * `MockIRCServer.channel_joined()`
-  * `MockIRCServer.mode_set()`
-  * `MockIRCServer.join()`
-  * `MockIRCServer.say()`
-  * `MockIRCServer.pm()`
+The `MockIRCServer` mock object from Sopel's `pytest` plugin implements
+`blocking` as a keyword-only argument in Sopel 8. This applies to:
+
+* `MockIRCServer.channel_joined()`
+* `MockIRCServer.join()`
+* `MockIRCServer.mode_set()`
+* `MockIRCServer.pm()`
+* `MockIRCServer.say()`
+
+To upgrade test cases using this mock, simply add `blocking=` to this parameter
+of the method call if it's missing.
 
 ### Moved API features