Add internal API docs for Binder

src/data/program_configuration.js

@@ -46,6 +46,30 @@ function packColor(color: Color): [number, number] {
     ];
 }
 
+/**
+ *  `Binder` is the interface definition for the strategies for constructing,
+ *  uploading, and binding paint property data as GLSL attributes.
+ *
+ *  It has three implementations, one for each of the three strategies we use:
+ *
+ *  * For _constant_ properties -- those whose value is a constant, or the constant
+ *    result of evaluating a camera expression at a particular camera position -- we
+ *    don't need a vertex buffer, and instead use a uniform.
+ *  * For data expressions, we use a vertex buffer with a single attribute value,
+ *    the evaluated result of the source function for the given feature.
+ *  * For composite expressions, we use a vertex buffer with two attributes: min and
+ *    max values covering the range of zooms at which we expect the tile to be
+ *    displayed. These values are calculated by evaluating the composite expression for
+ *    the given feature at strategically chosen zoom levels. In addition to this
+ *    attribute data, we also use a uniform value which the shader uses to interpolate
+ *    between the min and max value at the final displayed zoom level. The use of a
+ *    uniform allows us to cheaply update the value on every frame.
+ *
+ *  Note that the shader source varies depending on whether we're using a uniform or
+ *  attribute. We dynamically compile shaders at runtime to accomodate this.
+ *
+ * @private
+ */
 interface Binder<T> {
     property: string;
     statistics: { max: number };