Add a kdb5_util examples for old KDC upgrades

Add a kdb5_util examples for old KDC upgrades

It's a slightly less-contrived use case of the utility than the
other example, which reads more like a usage statement.
Give a motivating sentence before each example, and note that this
new example is not needed in the general upgrade case.

The need to dump/load for upgrades prior to 1.2 was documented in
the texinfo install guide, but not in any RST sources until now.

ticket: 7407