add Document for decorations only affect on live-preview-mode and not editing the line/content

[imshenshen]

https://marcus.se.net/obsidian-plugin-docs/editor/extensions/decorations#view-plugins

👉 shows both live preview mode, please add document about how to show 👉 only when the cursor not editing the content of what decorations want to change.

I think it is a common usage like # some heading shows in obsidian；

Live Preview delivers a cleaner writing experience by only displaying Markdown syntax around the cursor