gh-152361: Add module docstring to dataclasses

[zainnadeem786]

Summary

Add a module-level docstring to dataclasses so that help(dataclasses) displays a useful module description instead of relying only on the automatically generated documentation.

Changes

• Add a concise module docstring to Lib/dataclasses.py.
• No runtime behavior or public API changes.

Validation

Executed:

PCbuild\amd64\python_d.exe -c "import dataclasses; help(dataclasses)"
PCbuild\amd64\python_d.exe -m test test_dataclasses
git diff --check

All tests passed.

Closes gh-152361.

[aiwonderland]

wow

[picnixz]

I don't think it's necessary. Please in the future, don't open PRs until it's been acknowledged by a triager or core developer as a necessary feature.