Doc: Move WebEngineView property docs to webengine.qdoc

The docs do not currently get built, because docs for a QML type
need to reside in one file. Also edited the docs for grammar and
style.

Change-Id: Ic62f293e113d0e4bfe0497ce4a665a2a42971eb3
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>