Steps to repro:

1. Open Unity;
2. Create new project;
3. Add Terrain gameobject (GameObject -> 3D Object -> Terrain);
4. Select Terrain gameobject in the Hierarchy view;
5. Go to 'Terrain Settings' tab of the Terrain component;
6. Hover over any text field (e.g. Cast Shadows).

Expected result:
Tooltip with brief summary of the selected function is displayed.

Actual result:
Tooltip is not displayed. Inconsistent with other Unity components (e.g. 'Transform').
Also tooltips are not displayed in almost every tab of terrain component (except some fields) along with Terrain Collider component (for each field).
See attached screenshots - fields highlighted in green have tooltips, fields highlighted in red don't have ones.

Notes:
- Reproducible in: 5.4.0b7, 5.4.0b6, 5.3.3f1, 5.3.2f1, 5.3.1f1.
- Reproducible on both OSX 10.11.3 and Windows 10;
- Expected references for brief function summary could be found in Unity manual (for instance, Contents/Documentation/en/Manual/terrain-OtherSettings.html).