Expand and clarify XML_OUTPUT_FILE description in test encyclopedia

[eriwen]

This change adds a couple sentences to documentation based on my reading of https://github.com/bazelbuild/bazel/blob/master/tools/test/test-setup.sh#L178-L215

Motivation

To clarify the usage and default behavior of Bazel regarding test result XML output files.

This doc update might have saved me a few hours of pain — I'm hoping to save the next developer this time.

[eriwen]

Looks like the failing builds may be flaky and not caused by this documentation change (https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/11600#0181f40d-19f9-433f-8612-a899342e9bc0) — how should we handle this?

Is there something else you're waiting on me to do here?

[fmeum]

@eriwen You can rerun CI checks by amending without changes and then force pushing.