fix: misleading documentation on role name restrictions

[gardar]

corrects the documentation to reflect that role names can begin with an underscore (_). It also recommends reserving such names for internal roles within a collection, not for direct user invocation.

fixes: #2035

[ansible-documentation-bot]

Thanks for your Ansible docs contribution! We talk about Ansible documentation on Matrix at #docs:ansible.im if you ever want to join us and chat about the docs! We meet on Matrix every Tuesday. See the Ansible calendar for meeting details. We welcome additions to our weekly agenda items too. You can add the dawgs-meeting tag to a forum topic to bring it up at the next meeting.