How to reproduce:
1. Create a new Project or open an existing one
2. Create and select a new Animation Controller
3. Right-click on grid in Animator window > Create State > Empty and select the new node
4. In Inspector click documentation icon and observe opened page
5. Right-click on grid Create Sub-State Machine and select the new node
6. Select "Any State" node > repeat step 4
7. Select "Entry" node > repeat step 4
8. Select "Exit" node > repeat step 4

Actual Result: All cases open links to missing pages
Regular node opens https://docs.unity3d.com/Manual/class-AnimatorState.html
"Any State" node opens https://docs.unity3d.com/Manual/script-AnyStateNode.html
"Sub-State Machine" node opens https://docs.unity3d.com/Manual/class-AnimatorStateMachine.html
"Entry" node opens https://docs.unity3d.com/Manual/script-EntryNode.html
"Exit" node opens https://docs.unity3d.com/Manual/script-ExitNode.html

Expected Results: All cases open links to documentation pages
Empty and "Any State" nodes open https://docs.unity3d.com/Manual/class-State.html
"Sub-State Machine" node opens https://docs.unity3d.com/Manual/NestedStateMachines.html
"Entry" and "Exit" nodes open https://docs.unity3d.com/Manual/StateMachineTransitions.html

Reproducible on: 2017.4.36f1, 2018.4.15f1, 2019.2.19f1, 2019.3.0f5, 2020.1.0f20