user or group name: explain the supported format
    
    The commands ipa user-add or ipa group-add validate the
    format of the user/group name and display the following
    message when it does not conform to the expectations:
    invalid 'login': may only include letters, numbers, _, -, . and $
    
    The format is more complex, for instance '1234567' is an invalid
    user name but the failure is inconsistent with the error message.
    Modify the error message to point to ipa help user/group and add
    more details in the help message.
    
    Same change for idoverrideuser and idoverridegroup:
    The user/group name must follow these rules:
    - cannot contain only numbers
    - must start with a letter, a number, _ or .
    - may contain letters, numbers, _, ., or -
    - may end with a letter, a number, _, ., - or $
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2150217
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rafael Guterres Jeffman <rjeffman@redhat.com>
    Reviewed-By: Alexander Bokovoy <abbra@users.noreply.github.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>