[docs] add info on rebalancer's --report-only flag
[docs] add info on rebalancer's --report-only flag

Documented the --report-only command-line flag for the rebalancer
kudu CLI tool.  Documented the --output_replica_distribution_details
flag as well.

This is a follow-up to f6668ecac27492cbe6ba7682beff6d347650d689.

Change-Id: I80265eba61c347f70680636f32e0b80ee90ee59f
Reviewed-on: http://gerrit.cloudera.org:8080/11479
Tested-by: Kudu Jenkins
Reviewed-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

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