These descriptions are from the header of spark-daemon.sh

Author: Andrew Ash <andrew@andrewash.com>

Closes #2518 from ash211/SPARK-3663 and squashes the following commits:

058b257 [Andrew Ash] Complete hanging clause in SPARK_PID_DIR description
a17cb4b [Andrew Ash] Update docs for default locations per SPARK-4110
af89096 [Andrew Ash] SPARK-3663 Document SPARK_LOG_DIR and SPARK_PID_DIR