re-issueing new registration needs to be clearer

[Stuart Bishop]

'juju change-user-password' claims:

  This will invalidate any password or registration string
  that was previously issued, and issue a new registration string to be used with
  `juju register`.

It does not issue a new registration string, and there appears to be no way to generate it.

[Anastasia (anastasia-macmood)]

@stub,

The full text of this paragraph is:

```
A controller administrator can also reset the password for another user.
This will invalidate any password or registration string
that was previously issued, and issue a new registration string to be used with
`juju register`.

```

You need to use a --reset flag for command to work.

The help and the command is a bit obscure. Personal preference was and is to use a separate command. It'd be more intuitive!

I'll bring it up again :)

Thrilled you are using it \o/

[Stuart Bishop (stub)]

I think the behavior is fine here, and the docs just need tweaking. If it explicitly states that --reset invalidates the password and reissues the registration string, then people who think like me won't get confused.

[Anastasia (anastasia-macmood)]

PR against develop: https://github.com/juju/juju/pull/8692