Update documentation for QIODevice::readData().

readData() might be called with a maxSize of 0, regardless of the device
being buffered or not.

Task-number: QTBUG-28968
Change-Id: I666ed4d601341107722c86fe217f617ef748363d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
qdoc: Fix translation contexts.

Put all translations into the namespace QDoc and fix warnings
about invalid tr()-usage by removing the free tr()-function
from tr.h. Provide QCoreApplication::translate() for bootstrap
builds.

Change-Id: I2b6931188346f290e80e14b84adff8892d8a860f
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Fix a typo in the qdoc manual

Change-Id: I7eb6348562a18ce87e065950315dec3253a22aeb
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>