Added the documentation for macOS uiControl, or what little of it currently exists.

doc/00_index.md

@@ -4,3 +4,5 @@
 * [Initialization and the Main Loop](init-main.md)
 * [Event Handling](events.md)
 * [Controls](controls.md)
+* macOS
+** [Controls](darwin-controls.md)

doc/darwin-controls.md

@@ -0,0 +1,26 @@
+<!-- 10 june 2019 -->
+
+# Controls on macOS
+
+## Overview
+
+TODO
+
+## Reference
+
+### `uiControlOSVtable`
+
+```c
+typedef struct uiControlOSVtable uiControlOSVtable;
+struct uiControlOSVtable {
+	size_t Size;
+};
+```
+
+`uiControlOSVtable` describes the set of functions that control implementations on macOS need to implement. When registering your control type, you pass this in as a parameter to `uiRegisterControlType()`. Each method here is required.
+
+You are responsible for allocating and initializing this struct. To do so, you simply zero the memory for this struct and set its `Size` field to `sizeof (uiControlOSVtable)`. (TODO put this in a common place)
+
+Each method takes at least two parameters. The first, `c`, is the `uiControl` itself. The second, `implData`, is the implementation data pointer; it is the same as the pointer returned by `uiControlImplData(c)`, and is provided here as a convenience.
+
+Each method is named for the `uiDarwinControl` function that it implements. As such, details on how to implement these methods are documented alongside those functions. For instance, instructions on implementing `TODO()` are given under the documentation for `TOOD()`.