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".

docs/yaml/functions/project.yaml

@@ -67,15 +67,23 @@ kwargs:
     type: str | list[str]
     description: |
       Takes a string or array of strings describing the license(s) the code is under.
-      To avoid ambiguity it is recommended to use a standardized
-      license identifier from the [SPDX license list](https://spdx.org/licenses/).
-      Usually this would be something like `license : 'GPL-2.0-or-later'`, but if
-      the code has multiple licenses you can specify them as an array like this:
-      `license : ['proprietary', 'GPL-3.0-only']`. Note that the text is informal
-      and is only written to the dependency manifest. Meson does not do any license
-      validation, you are responsible for verifying that you abide by all licensing
-      terms. You can access the value in your Meson build files with
-      `meson.project_license()`.
+
+      This *should* be an [SPDX license expression](https://spdx.dev/ids/),
+      using the standardized license identifier from the [SPDX license
+      list](https://spdx.org/licenses/).  Usually this would be something like
+      `license : 'GPL-2.0-or-later'`. If there are multiple licenses you can use the
+      `AND` and `OR` operators to join them: `license : 'Apache-2.0 OR GPL-2.0'`.
+
+      For backwards compatibility reasons you can also pass an array of
+      licenses here. This is not recommended, as it is ambiguous: `license :
+      ['Apache-2.0', 'GPL-2.0-only']` instead use an SPDX espression: `license
+      : 'Apache-2.0 OR GPL-2.0-only'`, which makes it clear that the license
+      mean OR, not AND.
+
+      Note that the text is informal and is only written to the dependency
+      manifest. Meson does not do any license validation, you are responsible
+      for verifying that you abide by all licensing terms. You can access the
+      value in your Meson build files with `meson.project_license()`.
 
   subproject_dir:
     type: str