[CLI help] bootstrap can be improved re constraints

[Peter Matulis]

The pertinent help in `bootstrap` for constraints is:

===

--bootstrap-constraints (= "")
    Specify bootstrap machine constraints

--constraints (= "")
    Set model constraints

If '--bootstrap-constraints' is used, its values will also apply to any
future controllers provisioned for high availability (HA).

If '--constraints' is used, its values will be set as the default
constraints for all future workload machines in the model, exactly as if
the constraints were set with `juju set-model-constraints`.

===

I'd like to suggest the following wording:

===

--bootstrap-constraints (= "")
    Set constraints for controller machines

--constraints (= "")
    Set constraints for the 'controller' and 'default' models

If '--bootstrap-constraints' is used, its values will be used as constraints
for controllers only, including any future ones provisioned for high
availability.

If '--constraints' is used, its values will be used as constraints for every
machine in the 'controller' and 'default' models. This includes controllers
as well as non-controllers.

===

See also bug 1768308.

[Anastasia (anastasia-macmood)]

Marking as duplicate as this will be dealt with under bug # 1768308