Add some documentation for `localHostname`

Change-Id: Ib56a233731450ff1085bf66fe672a94cf865bd69
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217026
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>
diff --git a/sdk/lib/io/platform.dart b/sdk/lib/io/platform.dart
index bca141d..72053f5 100644
--- a/sdk/lib/io/platform.dart
+++ b/sdk/lib/io/platform.dart
@@ -115,6 +115,14 @@
   static String get operatingSystemVersion => _operatingSystemVersion;
 
   /// The local hostname for the system.
+  ///
+  /// For example:
+  ///   "mycomputer.corp.example.com"
+  ///   "mycomputer"
+  ///
+  /// Uses the platform
+  /// [`gethostname`](https://pubs.opengroup.org/onlinepubs/9699919799/)
+  /// implementation.
   static String get localHostname => _localHostname;
 
   /// Whether the operating system is a version of