Reflow G6 iterator docstrings

[WilliamK112]

Summary

• reflow G6 read/write iterator docstrings so each starts with a short summary sentence
• move longer details into separate paragraphs for cleaner API documentation rendering
• fix the touched sourceof typo in the reader string initializer docstring

Testing

• git diff --cached --check
• custom parser check for planarity/full/g6IterationUtils.pyx docstring summary shape
• cythonize(['planarity/full/g6IterationUtils.pyx'], build_dir=<temp>, force=True) with Cython 3.2.6 installed in a temporary directory

Refs #60