Add a note about training run for JIT snapshot

Closes #50615

R=bkonyi@google.com

Change-Id: I488c9f232ab5b0138d25a66006b8f69a05ea9c05
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311144
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

pkg/dartdev/lib/src/commands/compile.dart

@@ -442,6 +442,7 @@ class CompileCommand extends DartdevCommand {
     addSubcommand(CompileSnapshotCommand(
       commandName: CompileSnapshotCommand.jitSnapshotCmdName,
       help: 'to a JIT snapshot.\n'
+          'The executable will be run once to snapshot a warm JIT.\n'
           'To run the snapshot use: dart run <JIT file>',
       fileExt: 'jit',
       formatName: 'app-jit',