Extend spam checker to allow for multiple modules (#7435)

docs/sample_config.yaml

@@ -1867,10 +1867,17 @@ password_providers:
 #  include_content: true
 
 
-#spam_checker:
-#  module: "my_custom_project.SuperSpamChecker"
-#  config:
-#    example_option: 'things'
+# Spam checkers are third-party modules that can block specific actions
+# of local users, such as creating rooms and registering undesirable
+# usernames, as well as remote users by redacting incoming events.
+#
+spam_checker:
+   #- module: "my_custom_project.SuperSpamChecker"
+   #  config:
+   #    example_option: 'things'
+   #- module: "some_other_project.BadEventStopper"
+   #  config:
+   #    example_stop_events_from: ['@bad:example.com']
 
 
 # Uncomment to allow non-server-admin users to create groups on this server

docs/spam_checker.md

@@ -64,10 +64,12 @@ class ExampleSpamChecker:
 Modify the `spam_checker` section of your `homeserver.yaml` in the following
 manner:
 
-`module` should point to the fully qualified Python class that implements your
-custom logic, e.g. `my_module.ExampleSpamChecker`.
+Create a list entry with the keys `module` and `config`.
 
-`config` is a dictionary that gets passed to the spam checker class.
+* `module` should point to the fully qualified Python class that implements your
+  custom logic, e.g. `my_module.ExampleSpamChecker`.
+
+* `config` is a dictionary that gets passed to the spam checker class.
 
 ### Example
 
@@ -75,12 +77,15 @@ This section might look like:
 
 ```yaml
 spam_checker:
-  module: my_module.ExampleSpamChecker
-  config:
-    # Enable or disable a specific option in ExampleSpamChecker.
-    my_custom_option: true
+  - module: my_module.ExampleSpamChecker
+    config:
+      # Enable or disable a specific option in ExampleSpamChecker.
+      my_custom_option: true
 ```
 
+More spam checkers can be added in tandem by appending more items to the list. An
+action is blocked when at least one of the configured spam checkers flags it.
+
 ## Examples
 
 The [Mjolnir](https://github.com/matrix-org/mjolnir) project is a full fledged