Improve keytab documentation

Improve keytab documentation

In the k5srvutil man page, do not give the impression that arbitrary
new keys can be added to the keytab (requested by Dan Gillmor), since
only the new keys randomly generated by the KDC via 'k5srvutil change'
can be added to the keytab.  Reiterate the importance of running
k5srvutil delold after running k5srvutil change in the description of
k5srvutil change, as well as in the description of k5srvutil delold
itself.

In install_kdc.rst, mention using a separate keytab file when
generating a keytab on a KDC for use on another host.

[ghudson@mit.edu: squashed two commits, condensed commit message]

ticket: 8500