gh-129327: revise hashlib documentation to account for FIPS removing sha1 (GH-132729)

* gh-129327: revise hashlib documentation to account for FIPS removing sha1

More generally, the current documentation is a bit scattered, talking
about what terms are "equal" despite those terms not being very
interesting and given the term "secure hash", probably wrong (because
md5 and sha1 are not secure anymore).

Let's talk about cryptographically secure instead, and note that two of
them aren't. And then we can also link to the source for NIST going
through the removal process for SHA1.

* Add Gregors Suggestion

* Clean up

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Co-authored-by: Eli Schwartz <eschwartz@gentoo.org>