Shaders: mark the line shader as experimental.

I still expect the API to change slightly to accomodate for line
stipple or textured lines (such as possibly having different smoothness
alongside and across the segment), plus there are some TODOs that might
change how it behaves in certain corner cases.

src/Magnum/Shaders/LineGL.h

@@ -70,6 +70,8 @@ styles, and antialiased independently of MSAA being used or not.
 
 @image html shaders-line.png width=256px
 
+@experimental
+
 @requires_gl30 Extension @gl_extension{EXT,gpu_shader4}
 @requires_gles30 Requires integer support in shaders which is not available in
     OpenGL ES 2.0.