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]

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± library/struct.html