This is the better name, becuse this is not in particular about "docs".
It's about generating an XML with the information from the settings
meta data for nmcli.

We will do something similar with the libnm-core meta data.

It just feels nicer to be explicit about the filenames and
not rely on a specific naming.

Also, in meson we can directly pass the target as argument, which
expands to the filename but also adds a dependency.

Fixes: ab8fdb73e6be ('build: commit pre-generated "generate-docs-nm-settings-nmcli.xml" to git')

Otherwise, `ninja -C build uninstall` tries to delete "nmcli-completion",
when the file got renamed to "nmcli".

We depend on meson 0.47.2 already.

We require these, otherwise we can get a linker error about
_nm_utils_monotonic_timestamp_initialized symbol being undefined.