Documentation: explicit the expected meaning of a list of licenses

doc/pages/Manual.md

@@ -804,6 +804,7 @@ files.
 - <a id="opamfield-license">`license: [ <string> ... ]`</a>:
   The SPDX expression of the license(s) under which the source software is available
   (see http://spdx.org/licenses/). The [SPDX standard](https://spdx.github.io/spdx-spec/SPDX-license-expressions/) allows to define custom licenses if necessary using the `LicenseRef-your-custom-name` syntax (e.g. `license: "LicenseRef-My-Custom-Non-Commercial-License"`).
+  When several licenses are defined, the combination of them is equivalent to a single license expression separated by `AND`. For instance: `license: ["MIT" "ISC"]` is equivalent to `license: "MIT AND ISC"`. 
 
 - <a id="opamfield-homepage">`homepage: [ <string> ... ]`</a>,
   <a id="opamfield-doc">`doc: [ <string> ... ]`</a>,