[vm/ffi] Fix doc for Pointers being sent through SendPorts

https://dart-review.googlesource.com/c/sdk/+/350822 enabled sending
`Pointer`s via `SendPort`s, but omitted updating the doc.

TEST=doc fix only

Closes: #56236
CoreLibraryReviewExempt: FFI doc only.
Change-Id: Ib2b0a648856f328af310c460b3792709177807d8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/375820
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>
Auto-Submit: Daco Harkes <dacoharkes@google.com>

sdk/lib/isolate/isolate.dart

@@ -866,7 +866,6 @@ abstract interface class SendPort implements Capability {
   ///   - [Finalizable]
   ///   - [Finalizer]
   ///   - [NativeFinalizer]
-  ///   - [Pointer]
   ///   - [UserTag]
   ///   - `MirrorReference`
   ///