Fix failing SaveParameterFiles documentation test.

This commit fixes two issues.

  1. The output file was not being written in a cross-platform friendly
     way. We now use `os.path.expanduser` to select a writable file path
     on any platform.

  2. The usage example test was failing. This was caused by the faulty
     assumption that the output of SaveParameterFile is deterministic.
     The data it outputs is, but the actual ordering of elements in the
     XML isn't, so checking the raw contents of the XML file was an
     unreliable testing method. Instead we now rely on the unit test for
     SaveParameterFile and provide an example of the output to the user
     (and the pre-existing link to the parameter file wiki page).

Refs #10023