docs: mention quantity_point where appropriate

docs/use_cases/legacy_interfaces.rst

@@ -4,8 +4,9 @@ Working with Legacy Interfaces
 ==============================
 
 In case we are working with a legacy/unsafe interface we may be forced to
-extract a :term:`value of a quantity` with :func:`quantity::count()` and
-pass it to the library's output:
+extract the :term:`value of a quantity` with :func:`quantity::count()` or to
+extract the :term:`value of a quantity_point` with :func:`quantity_point::relative()`
+and pass it to the library's output:
 
 .. code-block::
     :caption: legacy.h
@@ -13,6 +14,7 @@ pass it to the library's output:
     namespace legacy {
 
     void print_eta(double speed_in_mps);
+    void set_path_position(double meter);
 
     } // namespace legacy
 
@@ -20,6 +22,7 @@ pass it to the library's output:
 
     #include "legacy.h"
     #include <units/physical/si/speed.h>
+    #include <units/quantity_point.h>
 
     using namespace units::physical;
 
@@ -34,6 +37,13 @@ pass it to the library's output:
       legacy::print_eta(quantity_cast<si::metre_per_second>(v).count());
     }
 
+    template <QuantityPoint QP>
+      requires Length<typename QP::quantity_type>
+    void set_path_position(QP p)
+    {
+      legacy::set_path_position(quantity_point_cast<si::metre>(p).relative().count());
+    }
+
 .. important::
 
     When dealing with a quantity of an unknown ``auto`` type please remember