Document '-paths-for-coverage.txt' for code coverage

### Description of the problem / feature request:

There is no documentation for `-paths-for-coverage.txt`.

### Feature requests: what underlying problem are you trying to solve with this feature?

In the process of using this 'feature' in https://github.com/bazelbuild/rules_scala/pull/1006 we just want to get some level of guarentee that this is not a hack and won't be broken anytime in the foreseeable future (or at least some level of transition plan to an alternative)

For reference the two places where this matters to us are:
https://github.com/bazelbuild/bazel/blob/567ca633d016572f5760bfd027c10616f2b8c2e4/src/java_tools/junitrunner/java/com/google/testing/coverage/JacocoCoverageRunner.java#L411
For parsing.

https://github.com/bazelbuild/bazel/blob/567ca633d016572f5760bfd027c10616f2b8c2e4/src/java_tools/junitrunner/java/com/google/testing/coverage/JacocoLCOVFormatter.java#L70
For using the set to find the source file.

Start with the referenced JacocoCoverageRunner.java location to understand how '-paths-for-coverage.txt' is parsed, then read JacocoLCOVFormatter.java to see how the paths are used to find source files. Document the file's format, behavior, and any available transition guidance so users can assess whether relying on it is supported.

Written by the indexing model from the issue text.