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/api.py

@@ -18,12 +18,23 @@ description:
   - This module can add, remove, update, query and execute arbitrary command in RouterOS via API.
 notes:
   - I(add), I(remove), I(update), I(cmd) and I(query) are mutually exclusive.
-  - I(check_mode) is not supported.
   - Use the M(community.routeros.api_modify) and M(community.routeros.api_find_and_modify) modules
     for more specific modifications, and the M(community.routeros.api_info) module for a more controlled
     way of returning all entries for a path.
 extends_documentation_fragment:
   - community.routeros.api
+  - community.routeros.attributes
+  - community.routeros.attributes.actiongroup_api
+attributes:
+  check_mode:
+    support: none
+  diff_mode:
+    support: none
+  platform:
+    support: full
+    platforms: RouterOS
+  action_group:
+    version_added: 2.1.0
 options:
   path:
     description: