Driver-Core: devtmpfs - remove EXPERIMENTAL and flush out the description

All major distros enable devtmpfs on recent systems, so remove
the EXPERIMENTAL flag, and make the description a bit more instructive.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>