powercap: make documentation reflect code

Fix up the documentation of the struct powercap_control_type members
to match the code.

Also fixup stray whitespace.

Signed-off-by: Amit Kucheria <amitk@kernel.org>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>