util: drop write_content_size=True
This is now the default in python-zstandard 0.9. While we're here,
also add a comment about the ability to drop frame magic to save
space.
Differential Revision: https://phab.mercurial-scm.org/D3199
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program