Metrics documentation outdated

HBASE-1687 fixes this supposedly but during the update of the metrics.xml there seemed to be a missed deletion of the last section describing the now obsolete OPTS addition. Especially see this note. Trivial fix by copying the version from trunk over.

I would also ask if we could change JMX_OPTS to HBASE_JMX_OPTS to be in line with other variables and when using this together with Edward's scripts who uses HADOOP_JMX_BASE. I asked Edward if he would be OK changing it to the matching HADOOP_JMX_OPTS. I also asked him to remove the copied section about editing the "hbase" start script in versions 0.20.1 or later.