gh-52008: document that absolute paths can break use_errno=True in ct…

[Das-Chinmay]

Loading a shared library via an absolute path (e.g. CDLL('/lib/libc.so.6'))
can cause use_errno=True to silently malfunction on Linux: dlopen may
return a separate instance of the library with its own errno variable, so
ctypes swaps the wrong one and get_errno() always returns 0.

Adds a .. note:: block to the use_errno parameter docs in
Doc/library/ctypes.rst warning about this and recommending the use of an
unqualified name or ctypes.util.find_library instead.

Fixes gh-52008

• Base: python/cpython:main
• Head: Das-Chinmay:fix-ctypes-use-errno-docs

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📚 Documentation preview 📚: https://cpython-previews--146504.org.readthedocs.build/

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This PR is stale because it has been open for 30 days with no activity.

[ZeroIntensity]

This issue isn't specific to absolute paths. The absolute paths problem was a symptom of older Ubuntu versions that shipped multiple libc libraries, which no longer happens.

The fundamental thing that we need to document is that use_errno only works if the library writes to the errno that ctypes can actually access (that is, the errno value from its own C runtime). I have a PR for this: #159239.