cmd/juju: update docs with info on placement directives

Also, bring add-machine docs up-to-date and a bit more cohesive.

Fixes https://bugs.launchpad.net/juju-core/+bug/1387421
Fix typos
Merge pull request #1013 from axw/lp1387421-juju-help-placement

cmd/juju: update docs with info on placement directives

Also, bring add-machine docs up-to-date and a bit more cohesive.

Fixes https://bugs.launchpad.net/juju-core/+bug/1387421