First pass at uniformed types, specs and docs

- spec public functions (skipping controllers, views, live views and
components)
- use types instead of explanations in docs
- remove redundant docs
- fix typos

lib/brave_search.ex

@@ -1,10 +1,12 @@
 defmodule BraveSearch do
   alias BraveSearch.API
 
+  @spec search_images(String.t(), keyword()) :: {:ok, [map()]} | {:error, term()}
   def search_images(query, opts \\ []) do
     API.search_images(query, opts, config())
   end
 
+  @spec download_image(String.t()) :: {:ok, binary()} | {:error, :download_failed}
   def download_image(url) do
     API.download_image(url, config())
   end