clarify cloud_init_userdata_templates group parameter

Author: sjpb

ansible/roles/cloud_init/README.md

@@ -10,7 +10,7 @@ Image and cloud environment supporting cloud-init.
 - `cloud_init_output_path`: Required. Path to output userdata files to.
 - `cloud_init_userdata_templates`: Optional list. Each element is a dict with keys/values as follows:
     - `module`: Required str. Name of cloud_init [module](https://cloudinit.readthedocs.io/en/latest/topics/modules.html)
-    - `group`: Optional str. Name of inventory group to which this config applies - if omitted it applies. This allows defining `cloud_init_userdata_templates` for group `all`.
+    - `group`: Optional str. Name of inventory group to which this config applies - if no group is specified then it applies to all groups. This allows defining `cloud_init_userdata_templates` for group `all`.
     - `template`: Jinja template for cloud_init module [configuration](https://cloudinit.readthedocs.io/en/latest/topics/modules.html).
 
   Elements may repeat `module`; the resulting userdata cloud-config file will will contain configuration from all applicable (by group) elements for that module.