docs: install: state that we do have an option for distro build

Brind the doc more up to date since we do have DISTRO=1 et al
available from the build system. Thus, no manual hand-tweaking
needs to be done, only in rare cases if really necessary.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
docs: install: recommend gnuplot for ifpps

ifpps supports a gnuplot-like output. Therefore, recommend the use
of gnuplot. It is not mandatory though.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>