selftests: arm64: Document what the SVE ptrace test is doing

Before we go modifying it further let's add some comments and output
clarifications explaining what this test is actually doing.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210913125505.52619-5-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>