Document the missing settings relevant to DataNode disk IO statistics

HDFS-11299 and HDFS-11339 introduced some new setting relevant to profiling hooks to expose latency statistics around DataNode disk IO. These settings should be intended for users but they are not documented. Totally three relevant settings:

1.dfs.datanode.enable.fileio.profiling
2.dfs.datanode.enable.fileio.fault.injection
3.dfs.datanode.fileio.profiling.sampling.fraction

Actually, only the setting dfs.datanode.enable.fileio.fault.injection don't need to be documented.