Show that multiple licenses are supported

[Leonidas-from-XIV]

The stanza is in the singular, but in fact supports more than one license. A lot of .opam.template files exist that add a license field to be able to specify multiple licenses where this is not strictly required.

[emillon]

I'm fine with the change, but did you see that "Multiple licenses may be specified" is explicitly stated below?
Also note that support for several licenses came only fairly recently (#6114) so these packages might have to bump the dune version to properly use this.

[Leonidas-from-XIV]

Yes, I saw the comment. I was writing #7059 (comment) and as I was analyzing which OPAM fields are in .opam.template files I saw a lot of license fields from templates, so I assumed that it might've been unclear to people that multiple licenses are allowed.

Thanks for the pointer to #6114, that (and the related linked issues) puts a new perspective on it that this has only been possible since dune 3.2, which explains why a lot of these files use license in .opam.template files.

I think the change is still useful, but maybe needs also a note on being available since dune 3.2.