Add a comment to RTL files for new contributors, take 2

[SergeyBiryukov]

Background: #30666

[41271] added a comment to minified files explaining that they are auto-generated, to help reduce some confusion for new contributors:

/*! This file is auto-generated */

The same should be done for RTL files, since they are also auto-generated during the build process, and should not be patched directly.

[swissspidy]

Is there a ticket for doing the same for JS files? Occasionally people try to fix things in minified Gutenberg scripts in core.

[SergeyBiryukov]

Replying to swissspidy:

Is there a ticket for doing the same for JS files?

Not that I know of, let's do it here :) 48424.diff​ should cover both JS and RTL CSS files.

[SergeyBiryukov]

In 46589:

Build/Test Tools: Add banner to RTL CSS and minified JS files.

Patches occasionally come in on generated files. We should be kind to new contributors and give them a hint that these files are auto-generated.

This is a follow-up to [41271], which added the banner to minified CSS files.

Fixes #48424. See #30666.