Fix up kdb5_util documentation

Fix up kdb5_util documentation

In kdb5_util.rst, reorder the main option summary to match the order
they are documented in below, and document the -x option.  Remove the
kdb5_util create -h switch case as 'h' has never been in the getopt
string.  Add the -r18 option to the kdb5_util dump and load option
summaries.  Reword the kdb5_util load -hash option.  Remove the
nonexistent kdb5_util load dbname parameter.

In database.rst, alter the example for loading a single principal to
use the dump principal filtering functionality, as that functionality
does not currently exist for load.

In the kdb5_util usage error message, reorder the main options to
match the order in the documentation and to fit within 79 columns.
Also add the -P option.

ticket: 8751 (new)
tags: pullup
target_version: 1.16-next