documentation: the license field syntax is poorly documented

[mseri]

I am not sure if this issue should be here. The license field documentation at https://opam.ocaml.org/doc/Manual.html#opamfield-license right now is very limited. We should at least mention also the WITH keyword and add an example using it, say "LGPL-2.0-or-later WITH OCaml-LGPL-linking-exception". If I am not mistaken there is also a mechanism for custom licenses, that should also be documented.

When opam 2.2 is released maybe we should mention there that it does use the spedx_licenses package to check the license correctness.

[mseri]

Last time this issue was mentioned is https://github.com/ocaml/opam/issues/5135 but it is not the first time that users ask us about the correct way to specify licenses.