sched/fair: Fix 'make xmldocs' warning caused by missing description

This patch fix following warning caused by missing description
"overload" in kernel/sched/fair.c

Warning(.//kernel/sched/fair.c:5906): No description found for
parameter 'overload'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1406518686-7274-1-git-send-email-standby24x7@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>