[offheap] Documentation for max off heap configuration

# Uncomment below if you intend to use off heap cache. For example, to allocate 8G of
# offheap, set the value to "8G".
# export HBASE_OFFHEAPSIZE=1G

This is what we have in hbase-env.sh now. We need more notes around this and some suggestion. May be more details in book also. This depends on whether we want L2 off heap BC as the default data cache in 2.0. (I would like to and will open a jira soon) Also if users use off heap MSLAB pool, then that also to be added up..
As of now we will use a ByteBufferPool which pool direct BBs and this is ON by default. The max size, this pool can keep is 2 MB * 2 * #handlers.
Will add more details as we turn ON other features.