Issue #16954: Add docstrings for ElementTree

Based on patch by David Lam
diff --git a/Misc/ACKS b/Misc/ACKS
index 8c4cf87..abb778c 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -677,6 +677,7 @@
 Erno Kuusela
 Ross Lagerwall
 Cameron Laird
+David Lam
 Thomas Lamb
 Jean-Baptiste "Jiba" Lamy
 Ronan Lamy