docs: clarify cload pairs compression support

[sapunyangkut]

Summary

• clarify that compression for path inputs is inferred from the filename extension
• limit the documented formats to those supported by pandas.read_csv and explicitly note that LZ4 is unsupported
• keep the generated CLI reference synchronized with the command docstring

Closes #410

Validation

• git diff --check
• parsed cload.py with Python's AST
• verified the cload pairs description is identical in the source docstring and docs/cli.rst

I could not run the full documentation build because the available package environment could not resolve a complete compatible cooler dependency set. This is a documentation-only change and does not alter runtime behavior.

This contribution was prepared with AI-assisted tooling. I reviewed the final diff and verified it against the implementation path and the maintainer's clarification in #410.