improve generated documentation for nm-settings

in man nm-settings we describe the individual properties. This gets generated from gtk-doc like https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/1916c55d3ae3d16b2137a2224fdb480f51da55df/src/libnm-core-impl/nm-setting-ip6-config.c#L862

Note that we have special tags like ---nmcli---. These tags get parsed by tools/generate-docs-nm-property-infos.py. There we support simple text (describtion:) and full fledged docbook XML (description-docbook:). Especially the latter support complex formatting (lists, enumerations).

Most properties don't overwrite this, and we just take the gtk-doc (see for example ipv6.dhcp-duid in man nm-settings-nmcli). Those gtk-docs get parsed by the tools/generate-docs-nm-settings-docs-gir.py script (resulting in src/libnm-client-impl/nm-settings-docs-gir.xml).

Later, tools/generate-docs-nm-settings-docs-merge.py merges src/libnm-client-impl/nm-property-infos-%.xml with src/libnm-client-impl/nm-settings-docs-gir.xml, and it goes on from there...

---

Anyway. Back to ipv6.dhcp-duid doc. You see, that it's very hard to read, because the entire text gets joined in one long paragraph. That is because tools/generate-docs-nm-settings-docs-gir.py's get_docs() strips out line breaks.

The task is to adjust get_docs(), so that we get still relatively nicely formatted documentation, preserving the individual paragraphs.

Try:

Xml_pretty() { 
    tidy --indent yes --indent-spaces 4 --indent-attributes yes --wrap-attributes yes --input-xml yes --output-xml yes "$@" 2> /dev/null
}

touch src/libnm-client-impl/NM-1.0.gir && make V=1 src/libnm-client-impl/nm-settings-docs-gir.xml

Xml_pretty src/libnm-client-impl/nm-settings-docs-gir.xml