common/algorithm: Add description comment indicating intended algorithms

Makes it explicit that the header is intended for iterator-based
algorithms that can ideally operate on any type.

src/common/algorithm.h

@@ -7,6 +7,11 @@
 #include <algorithm>
 #include <functional>
 
+// Algorithms that operate on iterators, much like the <algorithm> header.
+//
+// Note: If the algorithm is not general-purpose and/or doesn't operate on iterators,
+//       it should probably not be placed within this header.
+
 namespace Common {
 
 template <class ForwardIt, class T, class Compare = std::less<>>