system: clarify /dev management using devtmpfs + {mdev, eudev}

The "Dynamic using mdev" and "Dynamic using eudev" /dev management
methods are using devtmpfs, just like "Dynamic using devtmpfs only",
on top of which they add either mdev or eudev. But their names might
suggest they use mdev only or eudev only.

Clarify their descriptions by adding "devtmpfs +" to make it
unambiguous.

[Thomas: rewrap the paragraphs in the manual. It makes the diff a lot
less readable, but we actually want the result to be wrapped.]

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>