docs(tooltip): add overview (#2335)

* docs(tooltip): add overview

* add show/hide to overview

src/lib/tooltip/OVERVIEW.md

@@ -0,0 +1,35 @@
+The Angular Material tooltip provides a text label that is displayed with the user hovers 
+over or longpresses an element. 
+
+<!-- example(tooltip-overview) -->
+
+##Positioning
+
+The tooltip will be displayed below the element but this can be configured using the `mdTooltipPosition`
+input. 
+The tooltip can be displayed above, below, left, or right of the element. By default the position
+will be below. If the tooltip should switch left/right positions in an RTL layout direction, then
+the positions `before` and `after` should be used instead of `left` and `right`, respectively.
+
+| Position  | Description                                                                           |
+|-----------|---------------------------------------------------------------------------------------|
+| `above`   | Always display above the element                                                      |
+| `below `  | Always display beneath the element                                                    |
+| `left`    | Always display to the left of the element                                             |
+| `right`   | Always display to the right of the element                                            |
+| `before`  | Display to the left in left-to-right layout and to the right in right-to-left layout  |
+| `after`   | Display to the right in left-to-right layout and to the right in right-to-left layout |
+
+
+##Show and Hide
+
+The tooltip is immediately shown when the user's mouse hovers over the element and immediately 
+hides when the user's mouse leaves. A delay in showing or hiding the tooltip can be added through
+the inputs `mdTooltipShowDelay` and `mdTooltipHideDelay`.
+
+On mobile, the tooltip is displayed when the user longpresses the element and hides after a 
+delay of 1500ms.
+
+The tooltip can also be shown and hidden through the `show` and `hide` directive methods,
+which both accept a number in milliseconds to delay before applying the display change.
+