Make it clear that last_marked[] holds pointers to Lisp objects, not

the objects themselves.

etc/DEBUG

@@ -410,10 +410,13 @@ on the respective headers to remove the `:N' bitfield definitions
 
 ** Debugging problems which happen in GC
 
-The array `last_marked' (defined on alloc.c) can be used to display
-up to 500 last objects marked by the garbage collection process.  The
-variable `last_marked_index' holds the index into the `last_marked'
-array one place beyond where the very last marked object is stored.
+The array `last_marked' (defined on alloc.c) can be used to display up
+to 500 last objects marked by the garbage collection process.
+Whenever a Lisp object is marked by the garbage collector, it records
+the pointer to that object in the `last_marked' array.  The variable
+`last_marked_index' holds the index into the `last_marked' array one
+place beyond where the pointer to the very last marked object is
+stored.
 
 The single most important goal in debugging GC problems is to find the
 Lisp data structure that got corrupted.  This is not easy since GC