Close #14377: Add a new parameter to ElementTree.write and some module-level
serialization functions - short_empty_elements. It controls how elements
without contents are emitted.

Patch by Serhiy Storchaka. Feature initially proposed by Ariel Poliak.
Add Ariel to Misc/ACKS
Cleanup the docs ElementTree a bit and describe the default_namespace parameter. In the code, replace the old outdated Doxygen-ish comment above ElementTree.write by a proper docstring.