doc, zebra: Remove `keep_kernel_mode` from zebra

doc, zebra: Remove `keep_kernel_mode` from zebra

This code doees this:

a) Imagine ospf installs a route into zebra.  Zebra crashes and
we restart FRR.  If we are using the -k option on zebra than
all routes are re-read in, including this OSPF route.
b) Now imagine at the same time that zebra is starting backup
ospf on a different router looses a link to the this route.
c) Since zebra was run with -k this OSPF route is read back
in but never replaced and we now have a route pointing out
an interface to other routers that cannot handle it.

We should never allow users to implement bad options from zebra's
perspective that allow them to put themselves into a clear problem
state and additionally we have *absolutely* no mechanism to ever
fix that broken route without special human interaction.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>