Doc: Added more details to QIODevice::readAll()

Task-number: QTBUG-43666
Change-Id: Ibe56a72f9fa7a31a320aceda645fde9b08645271
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>

src/corelib/io/qiodevice.cpp

@@ -973,7 +973,7 @@ QByteArray QIODevice::read(qint64 maxSize)
 /*!
     \overload
 
-    Reads all available data from the device, and returns it as a
+    Reads all remaining data from the device, and returns it as a
     byte array.
 
     This function has no way of reporting errors; returning an empty