Code review comment for ~d-info-e/cloud-init:fix-user-group-doc

this one is harder than it should be.

_normalize_users ends up fixing the '-' to '_' in all those before
'distro.create_user' gets called. So either actually works.
Documentation and implementation in cloud-init have unfortunately been
inconsistent with '-' versus '_'.

We should absolutely strive to be consistent both in implementation
and in documentation.

So...
* I'm 98% certain that 'ssh-import-id' in the diff context and
  'ssh-authorized-keys' support either - or _. Lets just change everything to
  document '_' for consistency.
  Please fix those also.
* lets also update the in-module documentation in
  cloudinit/config/cc_users_groups.py to match (that is what ends up
  being documented at
  http://cloudinit.readthedocs.io/en/latest/topics/modules.html#users-and-groups