no-changed-when: add documentation

src/ansiblelint/rules/no_changed_when.md

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+# no-changed-when
+
+This rule checks that tasks return changes to results or conditions.
+Unless tasks only read information, you should ensure that they return changes in the following ways:
+
+- Register results or conditions and use the `changed_when` clause.
+- Use the `creates` or `removes` argument.
+
+You should use the `when` clause to run tasks only if a check returns a particular result.
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Does not handle any output or return codes
+      ansible.builtin.command: cat {{ my_file | quote }} # <- Does not handle the command output.
+```
+
+## Correct Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Handle shell output with return code
+      ansible.builtin.command: cat {{ my_file | quote }}
+      register: my_output # <- Registers the command output.
+      changed_when: my_output.rc != 0 # <- Uses the return code to define when the task has changed.
+```