Fix document config.active_storage.video_preview_arguments description. [skip ci]

[soartec-lab]

Summary

Removed default value and move arguments description.
config.active_storage.video_preview_arguments

Following the discussion of #39387, omit the default value config.active_storage.video_preview_arguments from the description of the settings. This is because it depends on config.load_defaults.

However, I think the rails 7.0 settings description is a good way to help developers understand, so I moved this description to the version 7.0 default settings section.

[jonathanhefner]

I agree that the explanation is helpful, and we should keep it. But it feels out of place in the long list of default values.

Since the discussion in #44029, I have proposed #44038 to prevent future confusion. With that PR, we could render:

(Source Code)

| Starting with version | The default value is |
| --------------------- | -------------------- |
| baseline              | `"-y -vframes 1 -f image2"` |
| 7.0                   | `"-vf 'select=eq(n\\,0)+eq(key\\,1)+gt(scene\\,0.015)"` <p><em>Select the first video frame, plus keyframes, plus frames that meet the scene change threshold.</em></p> `+ ",loop=loop=-1:size=2,trim=start_frame=1'"` <p><em>Use the first video frame as a fallback when no other frames meet the criteria by looping the first (one or) two selected frames, then dropping the first looped frame.</em></p> `+ " -frames:v 1 -f image2"` |

Or, perhaps even:

(Source Code)

| Starting with version | The default value is |
| --------------------- | -------------------- |
| baseline              | `"-y -vframes 1 -f image2"` |
| 7.0                   | `"-vf 'select=eq(n\\,0)+eq(key\\,1)+gt(scene\\,0.015)"`<sup><strong><em>1</em></strong></sup> <br> `+ ",loop=loop=-1:size=2,trim=start_frame=1'"`<sup><strong><em>2</em></strong></sup><br> `+ " -frames:v 1 -f image2"` <br><br> <ol><li>Select the first video frame, plus keyframes, plus frames that meet the scene change threshold.</li> <li>Use the first video frame as a fallback when no other frames meet the criteria by looping the first (one or) two selected frames, then dropping the first looped frame.</li></ol> |

In the mean time, I think we could just make an exception:

diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 5efe31b6bb..4554ace23f 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -1733,14 +1733,16 @@ The default is `:rails_storage_redirect`.

 Can be used to alter the way ffmpeg generates video preview images.

-By default, this is defined as:
+With `config.load_defaults 7.0`, the default is:

 ```ruby
 config.active_storage.video_preview_arguments = "-vf 'select=eq(n\\,0)+eq(key\\,1)+gt(scene\\,0.015),loop=loop=-1:size=2,trim=start_frame=1' -frames:v 1 -f image2"
 ```

+Which has the following behavior:
+
 1. `select=eq(n\,0)+eq(key\,1)+gt(scene\,0.015)`: Select the first video frame, plus keyframes, plus frames that meet the scene change threshold.
-2. `loop=loop=-1:size=2,trim=start_frame=1`: To use the first video frame as a fallback when no other frames meet the criteria, loop the first (one or) two selected frames, then drop the first looped frame.
+2. `loop=loop=-1:size=2,trim=start_frame=1`: Use the first video frame as a fallback when no other frames meet the criteria by looping the first (one or) two selected frames, then dropping the first looped frame.

 #### `config.active_storage.multiple_file_field_include_hidden`

[soartec-lab]

@jonathanhefner
Thank you for tell me. Your suggestion #44038 also looks good.
Regarding this PR, I have shown in the note that it is the default value of config.load_defaults 7.0 like the source code you provided.

could you review again ?

[jonathanhefner]

Looks good. I also added the last few lines of the diff (the part with "Which has the following behavior"). Thank you, @soartec-lab! 👍

Backported to 7-0-stable in 57edcf2.