In the jail man page (jail (8), NOT jail (2)) in the 6th paragraph or so of
the section "Setting up the Host Environment", it is stated that "a number
of services must be recompiled in order to run them in the host
environment". What precisely does this mean? Is there a way to recompile
(for example) nfsd (8)? If so, how? Where is this documented?

If the step is to make buildworld, etc., etc., why not say that? If there
are special things that somehow must be passed to these files prior to
compilation, what are those things?