v2025.12.22.00

Add documentation for pywatchman.WatchmanError

Summary:
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## Summary:
Added documentation string to the `WatchmanError` exception class in the pywatchman Python client library. The docstring describes:
- The purpose of the exception as the base class for watchman client errors
- When the exception is raised (communication failures, invalid responses, command errors)
- The attributes available on the exception (msg and cmd)

This improves code discoverability and helps developers understand how to properly use and handle watchman exceptions.
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Reviewed By: xavierd

Differential Revision: D87866042

fbshipit-source-id: b75a10db7926284a5cf542cc635c49b039709340