man: ip-address: document 15-char limit for LABEL

man: ip-address: document 15-char limit for LABEL

Trying to set a label longer than 15 characters returns an error:
 RTNETLINK answers: Numerical result out of range

Document the limit in the manpage.

Originally reported as a Debian bug:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=661886

Reported-by: Gabor Kiss <kissg@ssg.ki.iif.hu>
Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>