Updates to 'maintenance reason' spec

Updates the CLI description to include details of the 'ironic
node-set-maintenance' CLI that was added.

Updates the spec to mention deprecation of setting maintenance via
the node-update API.

Adds APIImpact flag, since the spec has two new methods to the REST API.

Change-Id: Ib88b57e9c274aa56836e009c9baefbf88e6b6aa1