Clarify the return type of Process.run when encoding is null

Being able to expect a Uint8List allows more efficient processing of
output.

Change-Id: Idf6fd6c581912e1958439d95790960f16fe549e6
CoreLibraryReviewExempt: This is a documentation-only change, describing existing behavior. It is VM-only (dart:io) and has approval from the owner of dart:io
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/383940
Commit-Queue: Sigurd Meldgaard <[email protected]>
Reviewed-by: Lasse Nielsen <[email protected]>
Reviewed-by: Brian Quinlan <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>

sdk/lib/io/process.dart

@@ -552,16 +552,18 @@ final class ProcessResult {
   /// value.
   final int exitCode;
 
-  /// Standard output from the process. The value used for the
-  /// `stdoutEncoding` argument to `Process.run` determines the type. If
-  /// `null` was used, this value is of type `List<int>` otherwise it is
-  /// of type `String`.
+  /// Standard output from the process.
+  ///
+  /// The value used for the `stdoutEncoding` argument to [Process.run]
+  /// determines the type. If `null` was used, this value is of type
+  /// [Uint8List] otherwise it is of type `String`.
   final stdout;
 
-  /// Standard error from the process. The value used for the
-  /// `stderrEncoding` argument to `Process.run` determines the type. If
-  /// `null` was used, this value is of type `List<int>`
-  /// otherwise it is of type `String`.
+  /// Standard error from the process.
+  ///
+  /// The value used for the `stderrEncoding` argument to [Process.run]
+  /// determines the type. If `null` was used, this value is of type
+  /// [Uint8List] otherwise it is of type [String].
   final stderr;
 
   /// Process id of the process.