Document skip and drop differences

They have similarity so its worth documenting what is different about them.

Author: romainbrenguier

src/util/range.h

@@ -343,6 +343,7 @@ struct ranget final
 
   /// Drop the first \p count elements.
   /// If the range has fewer elements, drops everything.
+  /// Similar to \ref skip but modifies the range in-place.
   void drop(std::size_t count)
   {
     std::size_t i = 0;
@@ -356,6 +357,7 @@ struct ranget final
   /// Return an new range containing the same elements except for the first
   /// \p count elements.
   /// If the range has fewer elements, returns an empty range.
+  /// Similar to \ref drop but returns a new range object.
   ranget<iteratort> skip(std::size_t count) const
   {
     ranget<iteratort> skip_range{begin(), end()};