docs: clarify isDesktopOnly means only runs on desktop

[mikedidomizio]

This PR clarifies the description of the isDesktopOnly property in the plugin manifest documentation.

This update makes the intent clearer by explaining that the property indicates the plugin only works on the desktop version of Obsidian.

This matches other places in Obsidian repos that mention this property.

I feel this is clearer from a plugin developer point of view, but happy to adjust the wording.

[joethei]

it should still be mentioned that this could be due to NodeJS/Electron API usage

[mikedidomizio]

Sure, I'm open to whatever the group lands on as the right fit.