Changed Doxygen single-line comment format

Single line comments starting with `///` are interpreted by
Microsoft Visual Studio as documentation containing XML code.

Therefore single line comments starting with `///<` unfortunately
cause a parsing error, as IntelliSense will assume the `<` is the
start of an XML tag. This is not the case, but unfortunately,
IntelliSense will still complain about the following space
character rather than displaying the raw string. This commit alters
all such comments to start with `//!<` instead, which prevents the
issue.

This fixes issue #1622.

include/SFML/Graphics/RectangleShape.hpp

@@ -99,7 +99,7 @@ private:
     ////////////////////////////////////////////////////////////
     // Member data
     ////////////////////////////////////////////////////////////
-    Vector2f m_size; ///< Size of the rectangle
+    Vector2f m_size; //!< Size of the rectangle
 };
 
 } // namespace sf