Have a common section on name properties

[pnorman]

Several tables have the name, name_en, and name_de properties defined. This works for now, but as #22 is solved and names gain in complexity, we should move the explanation on name properties to its own section.

This section should, in addition to defining properties, define the meaning of "named". Some layers contain geometries for "named ... polygons". It should be explicitly defined what named means, particularly for objects that have no name tag but do have a name:en and name:de tag.