How to trigger documentation popup in vscode

You also have, with VSCode 1.40 (Oct. 2019):

Definition Preview Hover from the keyboard

There is a new command Show Definition Preview Hover for better accessibility of the definition preview hover widget, which can be triggered by hovering a symbol with the mouse and pressing a modifier key dependent on the platform and configuration.

Previously, only the command Show Hover was provided, which is equivalent to hovering the mouse cursor over a symbol.
Now, with Show Definition Preview Hover, the detailed hover information can be shown via the keyboard.

This is the editor.action.showHover command. It is bound to cmdk cmdi by default.

Note: Shortcut works by holding down the cmd [ctrl in windows], then while holding press k then i

You can change the keyboard shortcut with a keybinding such as:

{
    "key": "cmd+k ctrl+space",
    "command": "editor.action.showHover",
    "when": "editorTextFocus"
}

The default shortcut for Trigger Parameter Hints is Ctrl+Shift+Space