Add docs for segmented control touch bar item

docs/api/structures/segmented-control-segment.md

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+# SegmentedControlSegment Object
+
+* `label` String - (Optional) The text to appear in this segment
+* `icon` NativeImage - (Optional) The image to appear in this segment
+* `enabled` Boolean - (Optional) Whether this segment is selectable. Default: true

docs/api/touch-bar-segmented-control.md

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+## Class: TouchBarSegmentedControl
+
+> Create a segmented control (a button group) where one button has a selected state
+
+Process: [Main](../tutorial/quick-start.md#main-process)
+
+### `new TouchBarSegmentedControl(options)`
+
+* `options` Object
+  * `segmentStyle` String - (Optional) Style of the segments:
+    * `automatic` - Default
+    * `rounded`
+    * `textured-rounded`
+    * `round-rect`
+    * `textured-square`
+    * `capsule`
+    * `small-square`
+    * `separated`
+  * `segments` [SegmentedControlSegment[]](structures/segmented-control-segment.md) - An array of segments to place in this control
+  * `selectedIndex` Integer (Optional) - The index of the currently selected segment, will update automatically with user interaction
+  * `change` Function - Called when the user selects a new segment
+    * `selectedIndex` - The index of the segment the user selected
+
+### Instance Properties
+
+The following properties are available on instances of `TouchBarSegmentedControl`:
+
+#### `touchBarSegmentedControl.segmentStyle`
+
+A `String` representing the controls current segment style.  Updating this value immediately updates the control
+in the touch bar.
+
+#### `touchBarSegmentedControl.segments`
+
+A `SegmentedControlSegment[]` array representing the segments in this control.  Updating this value immediately
+updates the control in the touch bar.  Updating deep properties inside this array **does not update the touch bar**.
+
+#### `touchBarSegmentedControl.selectedIndex`
+
+An `Integer` representing the currently selected segment.  Changing this value immediately updates the control
+in the touch bar.  User interaction with the touch bar will update this value automatically.