[Documentation - API] Add missing 'signal' parameter for /containers/:id/kill endpoint

Docker-DCO-1.1-Signed-off-by: Leo Unbekandt <leo.unbekandt@appsdeck.eu> (github: Soulou)
Merge pull request #4576 from Soulou/4549-doc-kill-signal-param

Add missing 'signal' parameter for kill endpoint