MonadCancel#uncancelable disrepancies

[neko-kai]

Given the description of behavior of Poll produced by MonadCancel#uncancelable in its scaladoc:

 /** ...
   * Masks can also be stacked or nested within each other. If multiple masks are active, all
   * masks must be undone so that cancelation can be observed. In order to completely unmask
   * within a multi-masked region the poll corresponding to each mask must be applied to the
   * effect, outermost-first.
   *
   * {{{
   *
   *   F.uncancelable { p1 =>
   *     F.uncancelable { p2 =>
   *       fa *> p2(p1(fb)) *> fc
   *     }
   *   }
   *
   * }}}
   *
   * The following operations are no-ops:
   *
   *   1. Polling in the wrong order
   *   1. Subsequent polls when applying the same poll more than once: `poll(poll(fa))` is
   *      equivalent to `poll(fa)`
   *   1. Applying a poll bound to one fiber within another fiber
   *
   * @param body
   *   A function which takes a [[Poll]] and returns the effect that we wish to make
   *   uncancelable.

ZIO.uninterruptibleMask as uncancelable does not satisfy this description:

1. In the code example, with ZIO, applying p1(fb) alone is enough to restore full interruptibility. The sentence "In order to completely unmask within a multi-masked region the poll corresponding to each mask must be applied to the effect, outermost-first." is not satisfied.
2. "Applying a poll bound to one fiber within another fiber is no-op" is not satisfied – restore functions produced by ZIO.uninterruptibleMask can migrate fibers and are not invalidated when used on another fiber.