Documentation: Add '\since' to doc for Qt::Edge

That enum got introduced in b6e9a8f21a

Change-Id: I2d69179624cecff63549bfee66bc68bb4b27af8c
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

src/corelib/global/qnamespace.qdoc

@@ -1735,6 +1735,8 @@
     \value LeftEdge   The left edge of the rectangle.
     \value RightEdge  The right edge of the rectangle.
     \value BottomEdge The bottom edge of the rectangle.
+
+    \since 5.1
 */
 
 /*!