Address review comments

Author: odersky

compiler/src/dotty/tools/dotc/typer/Deriving.scala

@@ -161,7 +161,7 @@ trait Deriving { this: Typer =>
      *
      *      implicit def derived$D(implicit ev_1: D[T_1], ..., ev_n: D[T_n]): D[C[Ts]] = D.derived
      *
-     *  See the body of this method for who to generalize this to typeclasses with more
+     *  See the body of this method for how to generalize this to typeclasses with more
      *  or less than one type parameter.
      *
      *  See test run/typeclass-derivation2 and run/derive-multi

docs/docs/reference/contextual/derivation.md

@@ -38,12 +38,7 @@ The generated typeclass instances are placed in the companion objects `Labelled`
 
 ### Derivable Types
 
-A trait or class can appear in a `derives` clause if
-
- - it has a single type parameter, and
- - its companion object defines a method named `derived`.
-
-These two conditions ensure that the synthesized derived instances for the trait are well-formed. The type and implementation of a `derived` method are arbitrary, but typically it has a definition like this:
+A trait or class can appear in a `derives` clause if its companion object defines a method named `derived`. The type and implementation of a `derived` method are arbitrary, but typically it has a definition like this:
 ```scala
   def derived[T] with Generic[T] = ...
 ```