add some more documentation of Is methods

core/src/main/scala/cats/evidence/Is.scala

@@ -21,15 +21,32 @@ abstract class Is[A, B] extends Serializable {
    */
   def substitute[F[_]](fa: F[A]): F[B]
 
+  /**
+    * `Is` is transitive and therefore values of `Is` can be composed in a
+    * chain much like functions. See also `compose`.
+    */
   @inline final def andThen[C](next: B Is C): A Is C =
     next.substitute[A Is ?](this)
 
+  /**
+    * `Is` is transitive and therefore values of `Is` can be composed in a
+    * chain much like functions. See also `andThen`.
+    */
   @inline final def compose[C](prev: C Is A): C Is B =
     prev andThen this
 
-  @inline final def from: B Is A =
+  /**
+    * `Is` is symmetric and therefore can be flipped around. Flipping is its
+    * own inverse, so `x.flip.flip == x`.
+    */
+  @inline final def flip: B Is A =
     this.substitute[? Is A](Is.refl)
 
+  /**
+    * Sometimes for more complex substitutions it helps the typechecker to
+    * wrap one layer of `F[_]` context around the types you're equating
+    * before substitution.
+    */
   @inline final def lift[F[_]]: F[A] Is F[B] =
     substitute[λ[α => F[A] Is F[α]]](Is.refl)