fix: add necessary comment to the preload file

[michalsn]

Description
This PR updates the sample preload.php file to include a short note in the instructions indicating that the debug echo statement should be removed before using the file in a real preloading setup. Preloading scripts are expected not to produce output, so this clarification helps prevent accidental usage of the sample with debug output left enabled.

Fixes#9821

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[paulbalandan]

I'm thinking if we should do the opposite instead: comment the echo line and add a note to uncomment that for debugging?

[neznaika0]

In my opinion, it's better to leave it - it's immediately clear that you need to update the paths and remove the output.
Although it looks like a bug after testing.

[michalsn]

I'm fine with changing it. Please see an alternative PR: #9825