libnm/docs: Preserve newlines in generate-docs-nm-settings-docs-gir.py

Previously it was hard to read long paragraphs.

Issue: #956

New output sample: https://gist.github.com/igo95862/deb5957ce5672ae7f646f28cf82fe1d7 (notice the paragraphs, these are native new lines)

It is also possible to use Python's textwrap to have paragraphs that are not based on source code.