Doc: Edited docs according to writing guidelines.

-added \since and added commands such as \readonly
-converted enum values to use \value
-edited language and other minor issues.
-only the main Qt WebEngine, the widget docs will be edited
 later.

Change-Id: Ifbb76a6472b5efede15daebc78d605b8b449d957
Task-number: QTBUG-40757
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>