docs: explain why we are silencing deprecation warnings

Author: nedbat

lab/show_pyc.py

@@ -183,6 +183,7 @@ def show_file(fname):
         print("Odd file:", fname)
 
 def main(args):
+    # "DeprecationWarning: co_lnotab is deprecated, use co_lines instead."
     warnings.filterwarnings("ignore", category=DeprecationWarning)
     if args[0] == '-c':
         show_py_text(" ".join(args[1:]).replace(";", "\n"))