Add explanatory comment to ZStackBottom sentinel

src/mudlib/zmachine/zstackmanager.py

@@ -94,6 +94,12 @@ class ZRoutine:
 
 
 class ZStackBottom:
+    """Sentinel object at the bottom of the call stack.
+
+    Includes stack and local_vars attributes so code that walks the call
+    stack can treat all frames uniformly without special-case checks for
+    the bottom sentinel.
+    """
     def __init__(self):
         self.program_counter = 0  # used as a cache only
         self.stack = []