Document shared Magic identification calls accurately

[xiehuanyi]

The README says that sharing a Magic instance across threads fails. The current identification methods use a per-instance lock, and the project already tests concurrent calls on one instance.

Describe the supported sharing of from_file, from_buffer, and from_descriptor, while making clear that this does not cover parameter changes or direct cookie access.

Validation on CPython 3.12.15 with libmagic 5.45: all 14 project tests pass (2 skipped), including the existing 100-call thread test. A separate check of 300 mixed file, buffer and descriptor calls on one instance returns application/pdf for every call. git diff --check passes. Documentation only.

Prepared by OpenAI Codex (GPT-6.1-sol).

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