docs/manual: makedevs only accepts numerical values

docs/manual: makedevs only accepts numerical values

Because of a limitation in how makedevs resolves names [0], usernames
and groupnames would be resolved with UIDs and GIDs from the host, not
the target.

Fixing makedevs would involve us rewriting a /etc/passwd and /etc/group
parser (not too complex, still not trivial).

Document that the user and group fields should only be numbers, not
names. Also, the manual is currently misleading, as an example indeed
uses names for the user and group fields. Fix that and expand the
explanations.

The mode field should also be numbers, not a rwx combo.

Slight typographical eye-candy about major/minor fields.

[0] it uses getpwnma(3) and getgrnam(3) to resolve names to IDs, and
those only resolve names on the host (i.e. there is no way to specify
a chroot-like feature)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>