Improve the description of the setting dfs.image.compress

In HDFS-1435, we introduced a new option to store fsimage compressed. And this can avoid that It consumes a lot of network bandwidth when SBN do uploads a new fsimage to ANN. When lots of network bandwidth was consumed, it will affect ANN to deal with normal RPC requests or sync edit logs.

This is a very useful setting when our fsimage file is very large. However current description of this setting is too simple, we can document this more.

<property>
  <name>dfs.image.compress</name>
  <value>false</value>
  <description>Should the dfs image be compressed?
  </description>
</property>