ParsedComment: Keep @usecase sections

ParsedComment used to work with the `expandedBody` of comments. The
`expandedBody` is the body of the comment after expansion, and without
the `@usecase` and `@define` tags. Because we want to show these, we use
`expanded` which keeps these tags.

Author: Duhemm

compiler/src/dotty/tools/dotc/util/ParsedComment.scala

@@ -23,8 +23,13 @@ class ParsedComment(val comment: Comment) {
    */
   private type Bounds = (Int, Int)
 
-  /** The content of this comment, after expansion if possible. */
-  val content: String = comment.expandedBody.getOrElse(comment.raw)
+  /**
+   * The content of this comment, after expansion if possible.
+   *
+   * Note that we use `expanded` instead of `expandedBody` to show `@usecase` and `@define`
+   * sections.
+   */
+  val content: String = comment.expanded.getOrElse(comment.raw)
 
   /** An index that marks all sections boundaries */
   private lazy val tagIndex: List[Bounds] = CommentParsing.tagIndex(content)