Doc: add a note about QRandomGenerator returning the sign bit set

Found this in a few uses of qrand() that assumed the result would be
non-negative.

Change-Id: Ia53158e207a94bf49489fffd14c7c029515cf42c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/global/qrandom.cpp

@@ -802,6 +802,15 @@ static Q_NEVER_INLINE void fill(void *buffer, void *bufferEnd)
 /*!
     Generates one 32-bit random value and returns it.
 
+    Note about casting to a signed integer: all bits returned by this function
+    are random, so there's a 50% chance that the most significant bit will be
+    set. If you wish to cast the returned value to int and keep it positive,
+    you should mask the sign bit off:
+
+    \code
+        int value = QRandomGenerator::get32() & std::numeric_limits<int>::max();
+    \endcode
+
     \sa get64(), getReal()
  */
 quint32 QRandomGenerator::get32()
@@ -814,6 +823,15 @@ quint32 QRandomGenerator::get32()
 /*!
     Generates one 64-bit random value and returns it.
 
+    Note about casting to a signed integer: all bits returned by this function
+    are random, so there's a 50% chance that the most significant bit will be
+    set. If you wish to cast the returned value to qint64 and keep it positive,
+    you should mask the sign bit off:
+
+    \code
+        qint64 value = QRandomGenerator::get64() & std::numeric_limits<qint64>::max();
+    \endcode
+
     \sa get32(), getReal(), QRandomGenerator64
  */
 quint64 QRandomGenerator::get64()