relink_light_sources comment

I remembered some more of the light source problem that led to the
panic() call that was fixed earlier today, but I'm not sure how
accurate this new comment is.  If it is accurate, the changes of
impossible() to panic() that the fix included won't necessarily
catch the old, apparently one-off, problem it describes.

src/light.c

@@ -496,6 +496,22 @@ relink_light_sources(boolean ghostly)
     unsigned nid;
     light_source *ls;
 
+    /*
+     * Caveat:
+     *  There has been at least one instance during to-be-3.7 development
+     *  where the light_base linked list ended up with a circular link.
+     *  If that happens, then once all the traversed elements have their
+     *  LSF_NEEDS_FIXUP flag cleared, the traversal attempt will run wild.
+     *
+     *  The circular list instance was blamed on attempting to restore
+     *  a save file which should have been invalidated by version/patch/
+     *  editlevel verification, but wasn't rejected because EDITLEVEL
+     *  didn't get incremented when it should have been.  Valid data should
+     *  never produce the problem and it isn't possible in general to guard
+     *  against code updates that neglect to set the verification info up
+     *  to date.
+     */
+
     for (ls = gl.light_base; ls; ls = ls->next) {
         if (ls->flags & LSF_NEEDS_FIXUP) {
             if (ls->type == LS_OBJECT || ls->type == LS_MONSTER) {