alloc/raw/linear: explain why begin/end members aren't const

alloc/raw/linear.hpp

@@ -99,8 +99,11 @@ namespace cruft::alloc::raw {
         size_t remain   (void) const;
 
     protected:
+        // The begin and end iterators should be constant but that interferes
+        // with move operators so we need to leave them mutable.
         std::byte *m_begin;
         std::byte *m_end;
+
         std::byte *m_cursor;
     };
 }