Document Timing Protocol units

[felix.the.red_gmail.com]

Summary

dhclient.conf(5): Document that for values in the Timing Protocol section,
the time parameter is in decimal seconds.
dhclient.conf(5): Document that time values in this section can also
be represented with octal or hexadecimal values.

From the original PR:
In its Protocol Timing section, the dhclient.conf(5) manpage lists
several time-related settings.
The unit in which the delays and timers are expressed is not specified.
Is the unit second, millisecond, minute, or anything else?
This should be part of the documentation.

PR:261682

Test Plan

Read sbin/dhclient/parse.c, and the function convert_num() to
understand how dhclient.conf(5) reads numbers for time-related settings.

[gbe]

LGTM