Documentation: clarify how to disable the generation of the /register route

[dogezen]

At the moment on the documentation for sign_in_route it isn't clear that you can omit the register_path option and that omitting it will ensure that /register path is not being generated.

Suggest amending the documentation paragraph to include a note about this:

`register_path` - the path under which to mount the password strategy's registration live-view. If not set, and registration is supported, registration will use a dynamic toggle and will not be routeable to. If a tuple {:unscoped, path} is provided, the registration path will not inherit the current route scope. If this option is omitted, then the `/register` route is not generated, in which case it is recommended to [override the sign in component](https://hexdocs.pm/ash_authentication_phoenix/ui-overrides.html#ashauthentication-phoenix-components-password) removing the register toggle text.

This could also link to the ash_authentication documentation on disabling registration: https://hexdocs.pm/ash_authentication/dsl-ashauthentication-strategy-password.html#authentication-strategies-password-registration_enabled?