fix(docs/build): MissingCoreLibraryException

Before this commit, `generateReferenceDocumentation` failed with
MissingCoreLibraryException even though it didn't use snipet checking.

It turned out that `sc:fail` causes the problem, but I do not know
exact reason for this.

Nothing but snippet compiler should use the `sc:fail` metadata.

Author: i10416

docs/_docs/reference/new-types/union-types-spec.md

@@ -156,7 +156,7 @@ The members of a union type are the members of its join.
 The following code does not typecheck, because method `hello` is not a member of
 `AnyRef` which is the join of `A | B`.
 
-```scala sc:fail
+```scala
 trait A { def hello: String }
 trait B { def hello: String }