Use attributes to document module capabilities; warn about command module's check_mode support (#118)

* Improve formatting.

* Add 'api' action group attribute.

* Document attributes.

* Known issue: command module declares to support check mode.

* Compatibility with older ansible-core releases.

* Fix typo.

* Improve docs.

* Add shortcuts for common combinations.

* Update changelogs/fragments/command-check_mode.yml

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

plugins/modules/command.py

@@ -20,6 +20,21 @@ description:
   - The module always indicates a (changed) status. You can use
     R(the changed_when task property,override_the_changed_result) to determine
     whether a command task actually resulted in a change or not.
+notes:
+  - The module declares that it B(supports check mode). This is a bug and will
+    be changed in community.routeros 3.0.0.
+extends_documentation_fragment:
+  - community.routeros.attributes
+attributes:
+  check_mode:
+    support: partial
+    details:
+      - The module claims to support check mode, but it simply always executes the command.
+  diff_mode:
+    support: none
+  platform:
+    support: full
+    platforms: RouterOS
 options:
   commands:
     description: