If one installs only the dhclient package, and not the dhcp package, the dhcp-options(5) man page isn't installed, because it resides in the dhcp package. However, the dhclient.conf(5) man page refers the user to the dhcp-options(5) man page multiple times. Although it's arguably a waste to create a "dhcp-common" package just to contain a man page and some /usr/share/doc files, having "broken" documentation links is also undesirable.
Including dhcp-options in dhclient as well.