[3.38] Document the --profile-startup Dart / Flutter CLI flags (#12272)

Co-authored-by: Parker Lougheed <[email protected]>

Author: derekxu16

src/content/tools/devtools/cpu-profiler.md

@@ -38,8 +38,15 @@ and Flame chart).
 To load all available CPU samples without manually
 recording and stopping, you can click **Load all CPU samples**,
 which pulls all CPU samples that the VM has recorded and
-stored in its ring buffer, and then displays those
-CPU samples in the profiler views.
+stored in its sample buffer and then
+displays those CPU samples in the profiler view.
+
+By default, the VM's sample buffer is used as a ring buffer,
+meaning that once it has become full, new samples start
+overwriting the oldest ones in the buffer.
+To make the VM instead discard all samples collected after
+the sample buffer has become full,
+pass the `--profile-startup` flag to `dart run` or `flutter run`.
 
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