[ISSUE] Amiguity of multi-license notation

[HiPhish <HiPhish@users.noreply.github.com>]

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New issue by HiPhish on void-packages repository

https://github.com/void-linux/void-packages/issues/48303

Description:
If a package is released under the terms of two licenses the manual says

> Multiple licenses should be separated by commas, Example: `GPL-3.0-or-later, custom:Hugware`.

However, this does not make it clear whether *either* of the license terms apply, or whether *both* apply. An example of a package where both licenses apply is `picom` which is under the terms of both the MPL-2.0 *and* MIT licenses.

This ambiguity could be avoided by using [SPDX expressions](https://spdx.github.io/spdx-spec/v2.3/SPDX-license-expressions/) to combine multiple licenses. These expressions can be arbitrarily complex and cover combinations of licenses (`AND` and `OR`) as well as exceptions (`WITH`) and grouping with parentheses. In the case of picom the SPDX expression is `MPL-2.0 AND MIT`. In fact, there is an SPDX file in the repo that contains this license: https://github.com/yshui/picom/blob/197b4bd396590cb5df61eb54ec6a1dadf1115a5d/LICENSE.spdx