Add some doctest examples for SemigroupK/MonoidK (#2077)

core/src/main/scala/cats/MonoidK.scala

@@ -26,6 +26,13 @@ import simulacrum.typeclass
 
   /**
    * Given a type A, create an "empty" F[A] value.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> MonoidK[List].empty[Long]
+   * res0: List[Long] = List()
+   * }}}
    */
   def empty[A]: F[A]
 

core/src/main/scala/cats/SemigroupK.scala

@@ -24,18 +24,45 @@ import simulacrum.typeclass
 
   /**
    * Combine two F[A] values.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> SemigroupK[List].combineK(List(1, 2), List(3, 4))
+   * res0: List[Int] = List(1, 2, 3, 4)
+   * }}}
    */
   @simulacrum.op("<+>", alias = true)
   def combineK[A](x: F[A], y: F[A]): F[A]
 
   /**
    * Given a type A, create a concrete Semigroup[F[A]].
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val s: Semigroup[List[Int]] = SemigroupK[List].algebra[Int]
+   * }}}
    */
   def algebra[A]: Semigroup[F[A]] =
     new Semigroup[F[A]] {
       def combine(x: F[A], y: F[A]): F[A] = self.combineK(x, y)
     }
 
+  /**
+   * "Compose" with a `G[_]` type to form a `SemigroupK` for `λ[α => F[G[α]]]`.
+   * Note that this universally works for any `G`, because the "inner" structure
+   * isn't considered when combining two instances.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> type ListOption[A] = List[Option[A]]
+   * scala> val s: SemigroupK[ListOption] = SemigroupK[List].compose[Option]
+   * scala> s.combineK(List(Some(1), None, Some(2)), List(Some(3), None))
+   * res0: List[Option[Int]] = List(Some(1), None, Some(2), Some(3), None)
+   * }}}
+   */
   def compose[G[_]]: SemigroupK[λ[α => F[G[α]]]] =
     new ComposedSemigroupK[F, G] {
       val F = self