add header doc comments to non-obvious util data structures

include/prepend_buffer.h

@@ -4,6 +4,10 @@
 #include <cstring>
 #include <span>
 
+// Buffer that grows in both directions from a midpoint. Payload is appended at
+// the back; each protocol layer then prepends its header in reverse order
+// (UDP, then IPv4, then Ethernet) without moving payload bytes. Lets the
+// network stack build outbound frames bottom-up with no copies or scratch.
 template <size_t N>
 class prepend_buffer {
     uint8_t m_buf[N];