chore(growth): add documentation and examples

lib/growth/calc/centile.ex

@@ -1,18 +1,39 @@
 defmodule Growth.Calc.Centile do
   @moduledoc """
-    measures =
-      [
-        [30, -1.7862, 16.9392, 0.1107],
-        [14, -1.3592, 20.4951, 0.12579],
-        [19, -1.6318, 16.049, 0.10038]
-      ]
-    Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
+  Calculate percentile for a given measurement and the fitted values of Box-Cox by age/height:
+
+  * power
+  * median
+  * coefficientof variation
+
+  This calculation is described in the [instructions provided by the World Health Organization](https://cdn.who.int/media/docs/default-source/child-growth/growth-reference-5-19-years/computation.pdf).
+
+  ## Examples:
+
+      iex> measures =
+      ...>   [
+      ...>     [30, -1.7862, 16.9392, 0.1107],
+      ...>     [14, -1.3592, 20.4951, 0.12579],
+      ...>     [19, -1.6318, 16.049, 0.10038]
+      ...>   ]
+      iex> Enum.map(measures, &apply(Growth.Calc.Centile, :compute, &1))
+      [:na, :na, 19.0]
+
   """
 
   alias Growth.Calc.ZScore
 
-  # TODO: (jpd) add documentation and typespecs
+  @spec compute(number(), number(), number(), number()) :: number()
+  @doc """
+  Compute the centile for a given measurement (`y`) and the Box-Cox values: power (`l`), median (`m`), and
+  coefficient of variation (`s`).
 
+  ## Examples:
+
+      iex> Growth.Calc.Centile.compute(19, -1.6318, 16.049, 0.10038)
+      19.0
+
+  """
   def compute(y, l, m, s) do
     zscore = ZScore.raw(y, l, m, s)