Document line shunt susceptance in lines_df

[sapunyangkut]

Description

Add the missing b entry to the Topology.lines_df docstring, documenting line shunt susceptance in Siemens and the total value for parallel lines. This follows the actual COLUMNS["lines_df"] order and avoids the issue's outdated suggestion that b is always zero; the current implementation can compute it from a line type's C_per_km.

Fixes #653

Type of change

• Bug fix (non-breaking documentation correction)

Checklist

• The docstring now lists every lines_df column in implementation order.
• git diff --check, Python compilation, AST parsing, and an exact docstring-column coverage assertion pass.
• No runtime code, type hints, dependencies, tests, or new features were added, so no package or whatsnew entry is needed.
• pre-commit and Sphinx were unavailable in the local environment; repository CI is requested for those checks.
• The two targeted project tests could not start locally because tests/conftest.py imports unavailable matplotlib.

AI assistance

This change was prepared with AI assistance and submitted through an automated contribution workflow. The final wording and diff were checked against the pinned eDisGo source, the current data-model documentation, and the issue acceptance criteria.