Doc: Added information about WebEngine

Removed a few webkit-specific sentences that are not
relevant at this point where the two web engines are
being introduced.

Change-Id: I30a6df3493bf806b933098785049932e91ede203
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>