Add some doctest examples for Alternative methods (#2070)

I took the examples from the `alternative` syntax file and slightly
modified them to use the type class instance directly instead of the
syntax helper methods.

core/src/main/scala/cats/Alternative.scala

@@ -9,20 +9,50 @@ import simulacrum.typeclass
     * to accumulate all of the "interesting" values of the inner G, so
     * if G is Option, we collect all the Some values, if G is Either,
     * we collect all the Right values, etc.
+    *
+    * Example:
+    * {{{
+    * scala> import cats.implicits._
+    * scala> val x: List[Vector[Int]] = List(Vector(1, 2), Vector(3, 4))
+    * scala> Alternative[List].unite(x)
+    * res0: List[Int] = List(1, 2, 3, 4)
+    * }}}
     */
   def unite[G[_], A](fga: F[G[A]])(implicit FM: Monad[F], G: Foldable[G]): F[A] =
     FM.flatMap(fga) { ga =>
       G.foldLeft(ga, empty[A])((acc, a) => combineK(acc, pure(a)))
     }
 
-  /** Separate the inner foldable values into the "lefts" and "rights" */
+  /**
+   * Separate the inner foldable values into the "lefts" and "rights"
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l: List[Either[String, Int]] = List(Right(1), Left("error"))
+   * scala> Alternative[List].separate(l)
+   * res0: (List[String], List[Int]) = (List(error),List(1))
+   * }}}
+   */
   def separate[G[_, _], A, B](fgab: F[G[A, B]])(implicit FM: Monad[F], G: Bifoldable[G]): (F[A], F[B]) = {
     val as = FM.flatMap(fgab)(gab => G.bifoldMap(gab)(pure, _ => empty[A])(algebra[A]))
     val bs = FM.flatMap(fgab)(gab => G.bifoldMap(gab)(_ => empty[B], pure)(algebra[B]))
     (as, bs)
   }
 
-  /** Return ().pure[F] if `condition` is true, `empty` otherwise */
+  /**
+   * Return ().pure[F] if `condition` is true, `empty` otherwise
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> def even(i: Int): Option[String] = Alternative[Option].guard(i % 2 == 0).as("even")
+   * scala> even(2)
+   * res0: Option[String] = Some(even)
+   * scala> even(3)
+   * res1: Option[String] = None
+   * }}}
+   */
   def guard(condition: Boolean): F[Unit] =
     if (condition) pure(()) else empty