Improve interceptor/Email.block docs

What changed?
============

We improve the interceptor and Email.block docs.

In particular, we update the module doc in the Interceptor behaviour to
use the `Email.block` function rather than `Email.intercept` (which was
the function name in a previous iteration).

We also add docs to the `def block` function in `Bamboo.Email` so that
it has its own documentation.

README.md

@@ -262,8 +262,8 @@ examples.
 
 ## Interceptors
 
-It's possible to configure per Mailer interceptors. Interceptors allow
-to modify / intercept (block) email on the fly.
+It's possible to configure per Mailer interceptors. Interceptors allow you to
+modify or block emails on the fly.
 
 ```elixir
 # config/config.exs
@@ -273,7 +273,8 @@ config :my_app, MyApp.Mailer,
 end
 ```
 
-An interceptor must implement the `Bamboo.Interceptor` behaviour. To prevent email being sent, you can block it with `Bamboo.Email.block/1`.
+An interceptor must implement the `Bamboo.Interceptor` behaviour. To prevent
+email being sent, you can block it with `Bamboo.Email.block/1`.
 
 ```elixir
 # some/path/within/your/app/deny_list_interceptor.ex

lib/bamboo/email.ex

@@ -266,6 +266,16 @@ defmodule Bamboo.Email do
     %{email | attachments: [Bamboo.Attachment.new(path, opts) | attachments]}
   end
 
+  @doc ~S"""
+  Marks an email as blocked to prevent it from being sent out. This is meant to
+  be used from an interceptor. To learn more about interceptors, see the
+  `Bamboo.Interceptor` behaviour.
+
+  ## Example
+
+      iex> Bamboo.Email.block(%Bamboo.Email{blocked: false})
+      %Bamboo.Email{blocked: true}
+  """
   def block(email) do
     %{email | blocked: true}
   end

lib/bamboo/interceptor.ex

@@ -2,7 +2,8 @@ defmodule Bamboo.Interceptor do
   @moduledoc ~S"""
   Behaviour for creating an Interceptor.
 
-  An interceptor allow to modify / block an email before it is sent. To block an email, it must be marked as intercepted with `Bamboo.Email.intercept/1`.
+  An interceptor allows you to modify or block an email before it is sent. To
+  block an email, it must be marked as blocked with `Bamboo.Email.block/1`.
 
   ## Example
 
@@ -12,7 +13,7 @@ defmodule Bamboo.Interceptor do
 
         def call(email) do
           if email.to in @deny_list do
-            Bamboo.Email.intercept(email)
+            Bamboo.Email.block(email)
           else
             email
           end

test/lib/bamboo/email_test.exs

@@ -1,5 +1,6 @@
 defmodule Bamboo.EmailTest do
   use ExUnit.Case
+  doctest Bamboo.Email
 
   import Bamboo.Email