KVM: Update documentation to include detailed ENABLE_CAP description

We have an ioctl that enables capabilities individually, but no description
on what exactly happens when we enable a capability using this ioctl.

This patch adds documentation for capability enabling in a new section
of the API documentation.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>