Add doc comment to Pure trait.

Author: odersky

library/src/scala/caps/Pure.scala

@@ -6,6 +6,15 @@ import language.experimental.captureChecking
 /** A marker trait that declares that all inheriting classes are "pure" in the
  *  sense that their values retain no capabilities including capabilities needed
  *  to perform effects. This has formal meaning only under capture checking.
+ *
+ *  NOTE: If an upper bound is Pure, we check that an argument conforms to it only
+ *  in sources where capture checking is enabled. For instance,
+ *
+ *    def f[C <: Pure]()
+ *    f[Object]()
+ *
+ *  would give an error only under capture checking. Pure is also dropped in
+ *  upper bounds if it forms part of an &-type, or is under a type lambda.
  */
 trait Pure:
   this: Pure =>