The prompt to hold the Ctrl key to open the documentation shows up (without any item/block icon, just the question mark hovering over nothing), when hovering over many UI elements, blocks, or entities - so far, in a minimal test instance with no mods except for Ars Elemental 0.7.2.2 itself, Ars Nouveau 5.5.1, Curios API 9.2.2, and GeckoLib 4.7.3, I've seen it happen at least on:

• Buttons and sliders in Options menus (both Minecraft's own and the NeoForge mod menu), but only those that already have a tooltip
• Creative Mode tabs, as well as the Destroy Item slot
• Empty Curios slots, as well as the Toggle Visibility button (on both filled and empty slots)
• Source Relays of all kinds, and the Redstone Relay
• Source Jars
• Spell Turrets of all kinds
• Wixie Cauldrons and Siren Shrines (though not Whirlisprig Blooms)
• Drygmy Henges, though seemingly only after they've achieved enough progress to give the "Source Needed" tooltip as well?
• Whirlisprigs and Starbuncles (though not Wixies, Sirens, or Drygmies)
• Storage Lecterns (though not Bookwyrms themselves)
• Spell Sensors
• Display Cases
• Urns of Endless Waters

And with the addition of other mods, I've also seen it on entries in EMI's recipe index, as well as on Create components that either produce or consume mechanical power (though not those that simply transmit it). It also should be noted that, in the case of blocks, this only happens when placed in-world - not when they're in item form. As far as I can tell, almost every tooltip in the game that isn't specifically an item tooltip gets

Holding Ctrl in these contexts does not show the normal green-circle indicator, but nonetheless, after a brief moment, it opens up the documentation page for the Aquamancer's Set. (Hovering over an actual Aquamancer's Set item, while it [alongside the other elemental armors] doesn't get an icon in its documentation-prompt tooltip, it does show the circular indicator while Ctrl is being held.)

As far as I've thus been able to tell, this happens when hovering over/looking at anything that has a tooltip of its own, except for items.