distances: Improve the documentation of the hwloc-distances tool and of the API

Clarify things, add cross-references, add more details in the hwloc-distances manpage.
Can't point to the ACPI spec SLIT section directly because the section number
has changed in the past (added as 5.2.16 in 3.0, became 5.2.17 in 4.0).

Thanks to Dave Love for the suggestion.

Fixes #119