docs: strongly recommend the use of SPDX instead of array of license
docs: strongly recommend the use of SPDX instead of array of license

The array of licenses is not clear, where and SPDX expression using AND
and OR is very clear, take for example this: `['Apache', 'GPLv2']`. What
does that mean? Any Apache license you like and GPLv2? Using a valid
SPDX license identifier however makes it extremely clear what is meant:
`'Apache-2.0 OR GPL-2.0-only'`. It is very clear that you mean, "this is
Apache 2.0, however, you can use as GPL-2.0 for the purpose of linking
it into your GPL-2.0 project".

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