deprecated-command: add documentation

src/ansiblelint/rules/deprecated_command_syntax.md

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+# deprecated-command-syntax
+
+This rule identifies the use of shorthand (free-form) syntax as this is highly
+discouraged inside playbooks, mainly because it can easily lead to bugs that
+are hard to identify.
+
+While using the free-form from the command line is ok, it should never be used
+inside playbooks.
+
+# Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Perform chmod
+      ansible.builtin.command: creates=B chmod 644 A # <-- do not use shorthand
+```
+
+# Correct Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: localhost
+  tasks:
+    - name: Perform chmod
+      ansible.builtin.command: chmod 644 A
+      args:
+        creates: B
+```