No documentation for overriding dhclient config files
I found no information in the man files documenting that when dhcp=dhclient in NetworkManager.conf, NM will search for a variety of dhclient.*.conf files in a preferential order to merge in config options that it wants to use. I'm creating this issue in part because I don't have a good sense of where it should go.
Usually it would go in NetworkManager(8) in a section what documents all the files that NM uses. However, NetworkManager(8) doesn't include such a section. The closest man I see if NetworkManager.conf(5), but that is specific to NetworkManager.conf processing. On my system nm-system-settings.conf(5) is aliased to NetworkManager.conf(5), so perhaps a good place would be nm-dhclient-settings.conf(5). But I'm not sure there's enough to info warrant its own manpage.
I haven't looked in to it, but there might be similar documentation for dhcpcd, and perhaps something to document on the internal dhcp client. With all that a new manpage may be warranted.