Update docs with slot predicate validation (#12842)

* update docs with slot validation

* address review comments

* rephrase customization of utter_internal_error_rasa

docs/docs/concepts/flows.mdx

@@ -441,7 +441,7 @@ response will be used to fill the "account_type" slot.
 :::note
 
 The slot needs to be defined in the domain file. See the
-[domain documentation](./domain.mdx) for more information. Additionaly, the slot
+[domain documentation](./domain.mdx) for more information. Additionally, the slot
 should have fitting slot mappings.
 
 :::
@@ -483,6 +483,45 @@ flow is completed.
   scope: "flow"
 ```
 
+#### Slot validation
+
+You can define slot validation rules directly in your flow yaml file by adding a `rejections` property to any
+`collect_information` step. The `rejections` section is a list of mappings, where each mapping must have an
+`if` and an `utter` mandatory properties:
+- the `if` property is a condition written in natural language that is evaluated using the
+[pypred](https://github.com/armon/pypred) library.
+- the `utter` property is the name of the [response](./responses.mdx) the assistant will send if the condition evaluates
+to `True`.
+
+Here is an example:
+
+```rasa-yaml title="flows.yml"
+flows:
+  verify_eligibility:
+    description: This flow verifies if the user is eligible for creating an account.
+    name: verify eligibility
+    steps:
+      - id: "ask_age"
+        collect: "age"
+        rejections:
+          - if: age < 1
+            utter: utter_invalid_age
+          - if: age < 18
+            utter: utter_age_at_least_18
+        next: "ask_email"
+```
+
+Upon every validation failure, the assistant will automatically make another attempt to collect the information.
+The assistant will repeatedly ask for the slot until none of the rejection conditions pass. Once all rejections have
+evaluated to `False`, the assistant moves on to the `next` step with the collected value.
+
+If the predicate defined in the `if` property is invalid, the assistant will log an error and respond with the
+`utter_internal_error_rasa` response which by default is `Sorry, I'm having trouble understanding you right now.
+Please try again later.`. You can override the text message for `utter_internal_error_rasa` by adding this response with
+the custom text in your domain file. We recommend using end-to-end testing to ensure that your flows work as expected.
+
+If no rejections are defined, the assistant will fill the slot with the collected value and proceed to the next step.
+
 ### Link
 
 A `link` step is used to link to another flow. Here is an example: