Patch 1

[MauritzFunke]

Description

I didn't know how to edit existing shortcodes, clarified that in the corresponding documentation

Issues

Fixes https://forum.silverstripe.org/t/edit-htmleditor-image-template/9952

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[GuySartorelli]

There are a lot more commits here than just the one with your change. Can you please update your pull request to only include the relevant commit?

These instructions may help.