Add description for dfs.namenode.caching.enabled

Namenode config:
dfs.namenode.write-lock-reporting-threshold-ms=50ms
dfs.namenode.caching.enabled=true (default)

In fact, the caching feature is not used in our cluster, but this switch is turned on by default(dfs.namenode.caching.enabled=true), incurring some additional write lock overhead. We count the number of write lock warnings in a log file, and find that the number of rescan cache warnings reaches about 32%, which greatly affects the performance of Namenode.

We should set 'dfs.namenode.caching.enabled' to false by default and turn it on when we wants to use it.