ACCUMULO-3308 Expand docs on configuration of IMM.

Expand on how the minc threshold, IMM size and WAL size
must be altered together. Additionally, mention how
the HDFS block size for the WAL should be increased to 2GB
when the WAL size is increased over 2GB.
Merge branch '1.6'

Conflicts:
	docs/src/main/latex/accumulo_user_manual/chapters/administration.tex