Update documentation in nets modules

Update the documentation of all the modules in [src/ansys/edb/core/net](https://github.com/ansys/pyedb-core/tree/main/src/ansys/edb/core/net)

- [ ] differential_pair.py
- [ ] extended_net.py
- [ ] net.py
- [ ] net_class.py

Start by reviewing the documentation in differential_pair.py, extended_net.py, net.py, and net_class.py under src/ansys/edb/core/net. Identify the documentation gaps in each module and update all four files. The work is done when each listed module has complete, consistent documentation.

Written by the indexing model from the issue text.