Clarify `maintainer` field (documentation, semantics)

https://cabal.readthedocs.io/en/3.4/cabal-package.html#pkg-field-maintainer says:
> maintainer: address
>
> The current maintainer or maintainers of the package. This is an e-mail address to which users should send bug reports, feature requests and patches.

Issues:
- "maintainers" (plural) and "an e-mail address" (singular) are in slight conflict with each other
- __one__ email address often does not accommodates __several__ maintainers
- the content of this field is de facto not rendered as an email address on hackage
- the idea that people send patches via email has fallen a bit out of time

Suggestions:
- change the field type to _freeform_
- change the text to:

> The current maintainer or maintainers of the package. May contain e-mail addresses which can be used to communicate with the maintainer(s).