chroot.md: Update mount syntax for /dev, /proc, /sys

[ahesford]

Apparently mount accepts --rbind and --make-rslave in the same
invocation, so we can merge the two commands per mount into one.

[ericonr]

I could swear I read something about --make-rslave being run in a separate invocation for a reason.

[ahesford]

Apparently this requires some further investigation.

[ericonr]

Here https://unix.stackexchange.com/questions/120827/recursive-umount-after-rbind-mount

[ahesford]

The StackExchange answer and subsequent comments are not consistent with my experience. The first comment says it's OK to combine, but the second comment suggests that combining the two will make it non-recursive. However, if I do

mount --make-rslave --rbind /dev /mnt

on my running system, I definitely have /mnt, /mnt/pts and /mnt/shm.

[ericonr]

I have observed the same thing as you. Perhaps there are some edge cases?

[ahesford]

Given the possibility that this does something unexpected, I don't see value in changing the docs to shave off a few keystrokes.