get_packet_size: Rewrite out_file's param's purpose in docstring

lvsfunc/util.py

@@ -250,8 +250,9 @@ def get_packet_sizes(
     or a scene, and to process certain frames differently depending on how much bitrate is allocated to specific
     sections.
 
-    If `out_file` is set, the results will be written to a file. If you want to access just the packet sizes for
-    speed purposes, it's recommended to have the function create and read that file.
+    If `out_file` is set, the results will be written to a file. This file will be read in subsequent calls to save
+    time. This is useful when you're working with a large clip and you don't want to call ffprobe every time you
+    refresh the preview.
 
     If a Keyframes object is passed, additional scene-based frame props will be added. These are the min, max, and
     average packet sizes of a scene based on these Keyframes.