Document master key rollover

Add a new section to database.rst documenting the procedure for
rolling the master key.

(cherry picked from commit e4b5d426a1e1e00367cc44a9619535ab71b20393)

https://github.com/krb5/krb5/commit/3d919ac599695bb460bbc419a85a8d0fd3727c91
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 3d919ac599695bb460bbc419a85a8d0fd3727c91
Branch: krb5-1.12
 doc/admin/database.rst |   51 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 51 insertions(+), 0 deletions(-)