Update offline-plugins.asciidoc for offline support

[SebastianThorn]

The current documentation is misleading or wrong.

https://discuss.elastic.co/t/airgapped-install-of-plugin-fails-trying-to-access-artifacts-elastic-co/356781

Release notes

Updates documentation for offline-installations

What does this PR do?

Fix documentation regarding offline-installations of plugins

Why is it important/What is the impact to the user?

The current documentation is wrong/misleading or incomplete.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Related issues

https://discuss.elastic.co/t/airgapped-install-of-plugin-fails-trying-to-access-artifacts-elastic-co/356781

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