doc: Document using a single or alternative shell
doc: Document using a single or alternative shell

A single component can function as a shell in Cockpit, if the
caller wants to use an alternate shell or a single shell.

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