Add footnote warning to hashing a hash #876

Job	Run time
py311 on alpine	1m 28s
Python 3.7 on win32	3m 25s
Python 3.11 on win32	3m 23s
Python 3.7 on win64	2m 56s
Python 3.11 on win64	2m 50s
Python 3.7 on macOS	1m 12s
Python 3.11 on macOS	1m 32s
pep8,packaging on linux, Rust stable	1m 57s
mypy on linux, Rust stable	1m 12s
py37 on linux, Rust stable	1m 9s
py38 on linux, Rust stable	1m 3s
py39 on linux, Rust stable	1m 11s
py310 on linux, Rust stable	57s
py311 on linux, Rust stable	55s
pypy3 on linux, Rust stable	1m 21s
pypy3 on linux, Rust stable	1m 16s
py311 on linux, Rust 1.60.0	3m 10s
py311 on linux, Rust beta	1m 2s
py311 on linux, Rust nightly	1m 3s
all-green	2s
	33m 4s

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