[flutter_tools] fix documentation on default built ios (#56720)

The default for build for flutter build ios should be release to match the other build commands. This is correctly handled in the command, but the documentation is incorrect. Update the documentation and the default to reflect the more common case of building for a physical device.

packages/flutter_tools/lib/src/commands/build_ios.dart

@@ -22,7 +22,7 @@ class BuildIOSCommand extends BuildSubCommand {
   BuildIOSCommand({ @required bool verboseHelp }) {
     addTreeShakeIconsFlag();
     addSplitDebugInfoOption();
-    addBuildModeFlags(defaultToRelease: false);
+    addBuildModeFlags(defaultToRelease: true);
     usesTargetOption();
     usesFlavorOption();
     usesPubOption();
@@ -35,7 +35,8 @@ class BuildIOSCommand extends BuildSubCommand {
     addBuildPerformanceFile(hide: !verboseHelp);
     argParser
       ..addFlag('simulator',
-        help: 'Build for the iOS simulator instead of the device.',
+        help: 'Build for the iOS simulator instead of the device. This changes '
+          'the default build mode to debug if otherwise unspecified.',
       )
       ..addFlag('codesign',
         defaultsTo: true,