Add _xxhash module docstring

[ifduyue]

Add a module docstring to the _xxhash C extension so help(xxhash._xxhash) is no longer empty.

[codspeed-hq]

Merging this PR will not alter performance

✅ 148 untouched benchmarks
⏩ 66 skipped benchmarks¹

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_{Comparing fix/xxhash-module-docstring (68f1000) with master (c8d9a0e)}

Footnotes

1. 66 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩