doc: Document also 'start=previous-boot' system API
doc: Document also 'start=previous-boot' system API

deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,deryni/cockpit,mvollmer/cockpit