Mention k5login_authoritative in k5login docs

In particular, it is set by default.  This can lead to confusing
behavior wherein adding a k5login file removes a user's remote
access.

Make an example more concrete to account for this case.

(cherry picked from commit 8cdc21ef051f43ea8dcabf42540d5cff13b5adeb)

https://github.com/krb5/krb5/commit/b755149d6e220aabdc1b862746082505ad6c982d
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: b755149d6e220aabdc1b862746082505ad6c982d
Branch: krb5-1.12
 doc/user/user_config/k5login.rst |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)