v2.1.x Chore - Improve recv module documentation

[coderabbitai]

Background

This issue is created as requested in PR #371.

Objective

Document the private constants (with 'w' prefix) in the recv module docstrings.

Details

The multicast/recv.py module contains several private warning message constants that need to be explicitly documented in the module-level docstring:

• _w_prefix
• _w_example_code
• _w_advice
• _w_empty_join_warning
• _w_unspec_bind
• _w_non_multicast

These constants are used for warning messages in the joinstep function and related functionality.

URL References

• PR where this follow-up was requested: Implemented join step empty groups edge-case logic #371 (comment)