Scaladoc: improve rendering of function types

[Florian3k]

See scaladoc-testcases/src/tests/thisType.scala#L7 for example:

https://github.com/lampepfl/dotty/blob/57c2b665117e0d10d490677edd0f83e093f80b97/scaladoc-testcases/src/tests/thisType.scala#L7

This could be rendered without the x$1 and ()

[Florian3k]

Fixed in #20333