(u)alarm.3: Improve readability of setitmer(2) restrictions

Add separators to make the maximum number of (micro)seconds
in (u)alarm.3 more readable.

Sponsored by: Klara, Inc.
Obtained from: https://github.com/apple-oss-distributions/Libc
Differential Revision: https://reviews.freebsd.org/D49712
Reviewed by: markj