gh-71086: Document negative offset support in struct.pack_into() and unpack_from()

[serhiy-storchaka]

struct.pack_into() and struct.unpack_from() (and the corresponding
Struct methods) accept a negative offset, counting from the end of the
buffer, but this was never documented.

Document it in Doc/library/struct.rst and in the docstrings of all four
callables. Also reword the module-level unpack_from() docstring: the old
"The buffer's size, minus offset, must be at least calcsize(format)" is
inaccurate for a negative offset, so it now reads "The buffer must contain at
least calcsize(format) bytes starting at offset".

• Issue: struct.pack_into(), struct.unpack_from() don't document support for negative offsets #71086

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33560504 | 📁 Comparing c30d567 against main (adebb68)

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1 file changed

± library/struct.html

[miss-islington-app]

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @serhiy-storchaka, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1706d14a2a1f009f786b8d7599e53c168b0392e0 3.14

[bedevere-app]

GH-153699 is a backport of this pull request to the 3.15 branch.

[miss-islington-app]

Sorry, @serhiy-storchaka, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 1706d14a2a1f009f786b8d7599e53c168b0392e0 3.13

[bedevere-app]

GH-153753 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-153754 is a backport of this pull request to the 3.13 branch.