Clarify details in boundary layer

[joto]

There are several issues with the documentation of the boundaries layer:

1. admin_level property is defined as numeric. I believe this should be clarified as unsigned integer (see also #31)
2. The feature table allows only admin_level 2 or 4. But in the description of the disputed property it says "unset or between 2 and 4". Is it on purpose that level 3 is allowed here? That unset is allowed here?
3. The definition for the "disputed" property is hard to parse and with several ands and ors without braces, it is ambigous.
4. We should probably explain more clearly that processing for the boundaries layer has to look at both the tags of the ways and the relations and how they interact, especially for the disputed case.