sched/core: Add comment explaining force-idle vruntime snapshots

sched/core: Add comment explaining force-idle vruntime snapshots

[ Upstream commit 9359d9785d85bb53f1ff1738a59aeeec4b878906 ]

I always end up having to re-read these emails every time I look at
this code. And a future patch is going to change this story a little.
This means it is past time to stick them in a comment so it can be
modified and stay current.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200506143506.GH5298@hirez.programming.kicks-ass.net
Link: https://lkml.kernel.org/r/20200515103844.GG2978@hirez.programming.kicks-ass.net
Link: https://patch.msgid.link/20251106111603.GB4068168@noisy.programming.kicks-ass.net
Stable-dep-of: 79f3f9bedd14 ("sched/eevdf: Fix min_vruntime vs avg_vruntime")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>