Describe the "Licenses" field

Most Eclipse projects have a single license. This is generally easy to understand... the project code is received and distributed under the terms of that license, as are the binaries.

This is true for most, but not for all projects.

Some projects, like Eclipse Temurin, have a collection of licenses, and it's not particularly clear to an observer what they mean.

The gist is that project code is under some subset of these licenses. Licenses are assumed by some of our scripts to be combined in particular ways (e.g., when EPL-2.0 and GPL-2.0-only are both listed our scripts assume that the GPL license is secondary as described in the EPL-2.0). We're tracking a revision to these practices in eclipsefdn/helpdesk#1313 (moved).

In the meantime, I'm thinking that we can avoid some of the confusion by adding a description to the field.

The content of this open source project is received and distributed under the license(s) listed below. Some source code and binaries may be distributed under different terms. Specific license information is provided in file headers and in NOTICE files distributed with the project's binaries.