doc: Add documentation for password resets performed during integration tests

When ewaol-virtualization and ewaol-security are both enabled, the test suites
set the password of the test user account on the Guest VM during their
executions. This patch adds the relevant documentation.

Issue-Id: SCM-4421
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Ie4b6fe745d3ef0be63ab711e7718882180c3fe32