Adds documentation for monolog processors.

Adds both a short description for existing processors as well as a note
for the newly added TokenProcessor added in 3.4.

Replaces #8156

Author: Denis Brumann

logging/processors.rst

@@ -160,6 +160,16 @@ If you use several handlers, you can also register a processor at the
 handler level or at the channel level instead of registering it globally
 (see the following sections).
 
+Symfony's MonologBridge provides processors that can be registered inside your application.
+
+* :class:`Symfony\\Bridge\\Monolog\\Processor\\DebugProcessor` adds additional information useful for debugging like a timestamp or an error message to the record.
+* :class:`Symfony\\Bridge\\Monolog\\Processor\\TokenProcessor` adds information from the current user's token to the record namely username, roles and whether the user is authenticated.
+* :class:`Symfony\\Bridge\\Monolog\\Processor\\WebProcessor` overrides data from the request using the data inside Symfony's request object.
+
+.. versionadded:: 3.4
+
+    The :class:`Symfony\\Bridge\\Monolog\\Processor\\TokenProcessor` class was added in Symfony 3.4.
+
 Registering Processors per Handler
 ----------------------------------