Fix allowed value for :copycss:

[rkratky]

The Allowed values field for the copycss attribute erroneously says empty. Instead, it should say that the value needs to specify the location of the stylesheet (provided we don't want the stylesheet attribute to resolve relative to the base directory. This PR fixes that.

See also https://github.com/asciidoctor/asciidoctor/issues/300#issuecomment-26478893.

[mojavelinux]

Technically, empty is a valid value if you are using the default (built-in) stylesheet. But you are correct that if you are using a custom stylesheet, it needs to be the stylesheet you want to copy. So it's really both.

[rkratky]

Right, so perhaps it should say 'empty or ...'? Because saying just 'empty' is confusing -- it sounds as if nothing but empty is allowed.

[rkratky]

So, specifically, "Empty or the location of the custom stylesheet (if used)".

[mojavelinux]

You got it!

[mojavelinux]

Thanks @rkratky!