Clarify the license field of the package

[宋文武 <iyzsong@envs.net>]

Hello list, I have some questions about the 'license' of a package,
currently defined as:

    The license of the package; a value from ‘(guix licenses)’, or a
    list of such values.

1. It's the license of source files (guix build -S) or built binary
   files?

2. When its value is a list of multiple licenses, it's files under
   different licenses (eg: lib/*.so under LGPL, while bin/* under GPL),
   or files under one license select from choices?

My guess is that the license field is for source files since we can
disable binary substitutes, and list is used for files under different
licenses.

Does my guess is correct?  Thank you!