## What changes were proposed in this pull request?

core/src/main/scala/org/apache/spark/SparkContext.scala contains LOCAL_N_FAILURES_REGEX master mode, but this was never documented, so do so.

## How was this patch tested?

By using the Github Markdown preview feature.

Author: Maurus Cuelenaere <mcuelenaere@gmail.com>

Closes #16562 from mcuelenaere/patch-1.