Clarify maintainer field (documentation, semantics)

[andreasabel]

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).