add more primitive docs

Author: boydm

lib/scenic/primitive/line.ex

@@ -62,8 +62,11 @@ defmodule Scenic.Primitive.Line do
   def default_pin(data), do: centroid(data)
 
   # --------------------------------------------------------
+  @doc """
+  Returns a the midpoint of the line. This is used as the default pin when applying
+  rotate or scale transforms.
+  """
   def centroid(data)
-
   def centroid({{x0, y0}, {x1, y1}}) do
     {
       (x0 + x1) / 2,

lib/scenic/primitive/rectangle.ex

@@ -57,8 +57,11 @@ defmodule Scenic.Primitive.Rectangle do
   def default_pin(data), do: centroid(data)
 
   # --------------------------------------------------------
+  @doc """
+  Returns the centroid of the rectangle. This is used as the default pin when applying
+  rotate or scale transforms.
+  """
   def centroid(data)
-
   def centroid({width, height}) do
     {width / 2, height / 2}
   end

lib/scenic/primitive/rounded_rectangle.ex

@@ -81,8 +81,11 @@ defmodule Scenic.Primitive.RoundedRectangle do
   def default_pin(data), do: centroid(data)
 
   # --------------------------------------------------------
+  @doc """
+  Returns a the centroid of the rectangle. This is used as the default pin when applying
+  rotate or scale transforms.
+  """
   def centroid(data)
-
   def centroid({width, height, _}) do
     {width / 2, height / 2}
   end

lib/scenic/primitive/triangle.ex

@@ -60,8 +60,11 @@ defmodule Scenic.Primitive.Triangle do
   def default_pin(data), do: centroid(data)
 
   # --------------------------------------------------------
+  @doc """
+  Returns the centroid of the triangle. This is used as the default pin when applying
+  rotate or scale transforms.
+  """
   def centroid(data)
-
   def centroid({{x0, y0}, {x1, y1}, {x2, y2}}) do
     {
       (x0 + x1 + x2) / 3,