added note about c_float type

Solver_interface/include/CGAL/OSQP_quadratic_program_traits.h

@@ -40,7 +40,14 @@ namespace CGAL {
   library, which must be available on the system.
 
   \tparam FT
-  number type
+  number type that `FieldNumberType`
+
+  \cgalModifBegin
+  \note The `FT` type is provided for convenience. Internally, this FT type is converted
+  to `c_float` type that can be set either to `float` or `double`. By default, usually
+  the `double` type is used. After the optimization is complete, the `c_float` type is
+  converted back to `FT`. See more about `c_float` <a href="https://osqp.org/docs/interfaces/cc++#data-types">here</a>.
+  \cgalModifEnd
 
   \cgalModels `QuadraticProgramTraits`
 */