The View | Tool Windows | Documentation menu option appears only if you have configured the IDE to display code documentation in a tool window.
Out of the box, IntelliJ IDEA shows code documentation in a popup as you hover the mouse pointer over symbols in the editor. You can change the view to use the tool window by default: press Ctrl+Q to open the popup and then press Ctrl+Q again.
Switch to the previous or go to the next documentation page (for example, after clicking hyperlinks).
On macOS, you can also use the three-finger right-to-left and left-to-right swipe gestures.
F4
Navigate to the source code in the editor.
Open as PopupCtrl+Q: show code documentation in a popup instead of the tool window.
Auto-update from Source: when this option is enabled, the content of the tool window changes as you move the mouse pointer in the editor.
If the Show on Mouse Move option is disabled, place the caret at the symbol to see its documentation in the tool window.
Adjust Font Size…: display a slider for changing the font size.
View Mode: by default, the tool window is attached to the edge of the main window and is always visible. You can change the view mode, for example, to make it visible only when active or to detach it from the tool window bar.
Move to: select where to attach the tool window.
Resize: adjust the size of the tool window.
Remove from Sidebar: close the tool window. To reopen it, select View | Tool Windows | Documentation from the main menu.