Document more tokens that get provided by events

docs/eca/concepts/tokens.md

@@ -54,10 +54,19 @@ Most events implicitly provide some tokens under pre-defined names.
     and the user needs to find such tokens either in support channels, in the code or by using the debugging techniques
     described in the [Debugging](/eca/debugging) section.
 
+The tokens `user` and `current_user` are always available, they contain the current user entity of the request that
+triggered the event.
+
 When an entity related event gets triggered, e.g.
 [Presave content entity](/plugins/eca/content/events/content_entity_presave), then ECA automatically puts the related
 entity onto the token stack with the name `entity` and also under the name of the entity type, e.g. `node`, `user`, etc.
 
+When a form related event gets triggered, e.g.
+[Build form](/plugins/eca/form/events/form_form_build), then ECA automatically puts the related form onto the token 
+stack with the names `form` and `current_form`.
+
+Others will be documented as soon as the above feature got implemented.
+
 ### Tokens declared by actions
 
 Most action plugins make their result available as a token, and their configuration form allows you to define the token