Dottydoc doesn't support multiple `@usecase` sections

[Duhemm]

scaladoc allows several @usecase sections to exist in a doc comment:

  /**
   * @usecase def foo: String
   * @usecase def foo: Int
   */
  def foo(x: Int): Unit

It renders them as several different definitions in the docsite:

Currently, dottydoc will only show the first @usecase.

Dottydoc should do the same thing.