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/open_ai/completion.ex

@@ -4,4 +4,11 @@ defmodule OpenAI.Completion do
             temperature: 0.2,
             role: "user",
             model: "gpt-4o-mini"
+
+  @type t :: %__MODULE__{
+          content: String.t(),
+          temperature: float(),
+          role: String.t(),
+          model: String.t()
+        }
 end