Expand and clarify XML_OUTPUT_FILE description in test encyclopedia
Expand and clarify XML_OUTPUT_FILE description in test encyclopedia

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.

Closes #15863.

PiperOrigin-RevId: 481868440
Change-Id: I8f69f43a9923b55ae6cdf07d719ae3bda10b13e0

bazelbuild/bazel,katre/bazel,bazelbuild/bazel,katre/bazel,katre/bazel,katre/bazel,katre/bazel,bazelbuild/bazel,katre/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel