Update docs about rasa data validate flows checks (#12916)

* update docs about rasa data validate flows

* address review comments

* typo ci update, add more context

.typo-ci.yml

@@ -217,5 +217,6 @@ excluded_words:
   - HashiCorp
   - sqltrackerstore
   - wsgi
+  - pypred
 
 spellcheck_filenames: false

docs/docs/command-line-interface.mdx

@@ -23,7 +23,7 @@ The following commands are relevant to all assistants built with Rasa.
 | `rasa run actions`      | Starts an action server using the Rasa SDK.                                                        |
 | `rasa test e2e`         | Runs end-to-end testing fully integrated with the action server that serves as acceptance testing. |
 | `rasa data split nlu`   | Performs a 80/20 split of your NLU training data.                                                  |
-| `rasa data validate`    | Checks the domain, NLU and conversation data for inconsistencies.                                  |
+| `rasa data validate`    | Checks the domain, NLU, flows and conversation data for inconsistencies.                                  |
 | `rasa export`           | Exports conversations from a tracker store to an event broker.                                     |
 | `rasa evaluate markers` | Extracts markers from an existing tracker store.                                                   |
 | `rasa marker upload`    | Upload marker configurations to Analytics Data Pipeline                                            |
@@ -562,7 +562,7 @@ rasa data migrate --help
 
 ## rasa data validate
 
-You can check your domain, NLU data, or story data for mistakes and inconsistencies.
+You can check your domain, NLU data, flows or story data for mistakes and inconsistencies.
 To validate your data, run this command:
 
 ```bash
@@ -574,6 +574,7 @@ identical training examples.
 The validator also checks if you have any stories where different assistant actions follow from the same
 dialogue history. Conflicts between stories will prevent a model from learning the correct
 pattern for a dialogue.
+To learn more about the checks performed by the validator on flows, continue reading in the [next section](./command-line-interface.mdx#validate-flows).
 
 :::info Searching for the `assistant_id` key introduced in 3.5
 
@@ -585,6 +586,25 @@ key is missing or if the default value has not been changed.
 If you pass a `max_history` value to one or more policies in your `config.yml` file, provide the
 smallest of those values in the validator command using the `--max-history <max_history>` flag.
 
+### Validate flows
+
+The validator will perform the following checks on flows:
+- determine whether flow names or descriptions are unique after stripping punctuation
+- verify whether natural language conditions used in [branch steps](./concepts/flows.mdx#branching-conditions) or `collect` step [rejections](./concepts/flows.mdx#slot-validation) are valid `pypred` expressions
+- determine whether slots used in flows are defined in the domain
+- disallow `list` slots from being used in flows `collect` steps: CALM supports only filling slots with values of type `int`, `string` or `bool` in flows.
+- disallow `dialogue_stack` internal slot from being used in flows
+
+For every failure, the validator will log an error and exit the command with exit code 1.
+
+You can validate flows only by running this command:
+
+```bash
+rasa data validate flows
+```
+
+### Validate story structure
+
 You can also validate only the story structure by running this command:
 
 ```bash