Cleanup in EntityContent.php

* Added documentation for how EntityContent::equal behaves since this is not clear from just
  looking at Content::equals.
* Changed code of EntityContent::equal to be more self-explanatory
* Changed some comments/documentation to fit into the 100chars per line limit

Change-Id: If10c2877c9d4de83880c94e0a79f00a24aea1e91
Merge "Cleanup in EntityContent.php"