Doc: Clarify QBitArray::fill() documentation

Fix a minor mistake in the the function parameter documentation,
and add a code snippet.

Task-number: QTBUG-39782
Change-Id: Ia5d88a983ad683ae5bde9f332d51adc4afda77a8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>