Fix module documentation

Fix 'lib/_constants.py.in' documentation to be consistent with other
modules.
Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

lib/_constants.py.in

@@ -3,9 +3,8 @@
 
 """Automatically generated constants for Python
 
-This file is autogenerated by the build process.
-For any changes you need to re-run ./configure (and
-not edit by hand).
+Note that this file is autogenerated with @lib/_constants.py.in@ as a
+header.
 
 """