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>

src/corelib/doc/snippets/code/src_corelib_tools_qbitarray.cpp

@@ -174,3 +174,10 @@ a[0] = 1; a[1] = 0; a[2] = 1;   // a: [ 1, 0, 1 ]
 b[0] = 1; b[1] = 1;             // b: [ 1, 1 ]
 c = a ^ b;                      // c: [ 0, 1, 1 ]
 //! [14]
+
+//! [15]
+QBitArray ba(4);
+ba.fill(true, 1, 2);            // ba: [ 0, 1, 0, 0 ]
+ba.fill(true, 1, 3);            // ba: [ 0, 1, 1, 0 ]
+ba.fill(true, 1, 4);            // ba: [ 0, 1, 1, 1 ]
+//! [15]

src/corelib/tools/qbitarray.cpp

@@ -290,11 +290,18 @@ void QBitArray::resize(int size)
 /*!
     \overload
 
-    Sets bits at index positions \a begin up to and excluding \a end
+    Sets bits at index positions \a begin up to (but not including) \a end
     to \a value.
 
-    \a begin and \a end must be a valid index position in the bit
-    array (i.e., 0 <= \a begin <= size() and 0 <= \a end <= size()).
+    \a begin must be a valid index position in the bit array
+    (0 <= \a begin < size()).
+
+    \a end must be either a valid index position or equal to size(), in
+    which case the fill operation runs until the end of the array
+    (0 <= \a end <= size()).
+
+    Example:
+    \snippet code/src_corelib_tools_qbitarray.cpp 15
 */
 
 void QBitArray::fill(bool value, int begin, int end)