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.