Document the return value of any and every for empty values.

Closes https://github.com/dart-lang/sdk/pull/52847

GitOrigin-RevId: f5382101762582c7d865833e42d9af39fc0a6131
Change-Id: Iafeb53a7e583e0309d08d9dffe3803b7f772b70d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/312380
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>

sdk/lib/core/iterable.dart

@@ -412,6 +412,7 @@ abstract mixin class Iterable<E> {
   ///
   /// Checks every element in iteration order, and returns `false` if
   /// any of them make [test] return `false`, otherwise returns `true`.
+  /// Returns `true` if the iterable is empty.
   ///
   /// Example:
   /// ```dart
@@ -465,6 +466,7 @@ abstract mixin class Iterable<E> {
   ///
   /// Checks every element in iteration order, and returns `true` if
   /// any of them make [test] return `true`, otherwise returns false.
+  /// Returns `false` if the iterable is empty.
   ///
   /// Example:
   /// ```dart