Doc: Edited "Integrating Web Content" page

-made distinction between Qt WebKit and Qt WebKit Widget clearer
-corrected module names: "QtWebKit" to "Qt WebKit"
-replicated the information into the "User Interfaces" page

Change-Id: Ic1dd742dde2c567bb008a4e4b2f701521b30c79f
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>