[sysprof] capture: add comment about frame tracking

[Christian Hergert <chergert src gnome org>]

commit b351e1a50a57d6b7e742d72c39c6f253c2071623
Author: Christian Hergert <chergert redhat com>
Date:   Tue Feb 11 18:07:31 2020 -0800

    capture: add comment about frame tracking

 src/libsysprof-capture/sysprof-capture-reader.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/libsysprof-capture/sysprof-capture-reader.c b/src/libsysprof-capture/sysprof-capture-reader.c
index 4774e3f..c6c4cc3 100644
--- a/src/libsysprof-capture/sysprof-capture-reader.c
+++ b/src/libsysprof-capture/sysprof-capture-reader.c
@@ -419,6 +419,10 @@ sysprof_capture_reader_peek_frame (SysprofCaptureReader *self,
 
   sysprof_capture_reader_bswap_frame (self, frame);
 
+  /* In case the capture did not update the end_time during normal usage,
+   * we can update our cached known end_time based on the greatest frame
+   * we come across.
+   */
   if (frame->time > self->end_time)
     self->end_time = frame->time;