[SPARK-8405] [DOC] Add how to view logs on Web UI when yarn log aggregation is enabled

Some users may not be aware that the logs are available on Web UI even if Yarn log aggregation is enabled. Update the doc to make this clear and what need to be configured.

Author: Carson Wang <carson.wang@intel.com>

Closes #7463 from carsonwang/YarnLogDoc and squashes the following commits:

274c054 [Carson Wang] Minor text fix
74df3a1 [Carson Wang] address comments
5a95046 [Carson Wang] Update the text in the doc
e5775c1 [Carson Wang] Update doc about how to view the logs on Web UI when yarn log aggregation is enabled