[PATCH v2] sched/deadline: document new sched_getattr() feature for retrieving current parameters for DEADLINE tasks

Compared to the initially submitted documentation patch, this
version addresses the issue highlighted by Juri of the wrong wrapping
of the commit message, and the one found by the chatbot of the wrong
use of quotes around the flags parameter. I'm also adding "v2" in
the subject line, as requested by Randy.