Add nodoc to Mail module [ci-skip]

`Mail` gem is an external gem and all methods extended by Rails
are undocumented. However it's listed in API document page.
It serves no purposes so it should be removed.

Author: okuramasafumi

actionmailbox/lib/action_mailbox/mail_ext/address_equality.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-module Mail
-  class Address
+module Mail # :nodoc: all
+  class Address # :nodoc: all
     def ==(other_address)
       other_address.is_a?(Mail::Address) && to_s == other_address.to_s
     end

actionmailbox/lib/action_mailbox/mail_ext/address_wrapping.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-module Mail
+module Mail # :nodoc: all
   class Address
     def self.wrap(address)
       address.is_a?(Mail::Address) ? address : Mail::Address.new(address)

actionmailbox/lib/action_mailbox/mail_ext/addresses.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
-module Mail
-  class Message
+module Mail # :nodoc: all
+  class Message # :nodoc: all
     def from_address
       address_list(header[:from])&.addresses&.first
     end

actionmailbox/lib/action_mailbox/mail_ext/from_source.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-module Mail
+module Mail # :nodoc: all
   def self.from_source(source)
     Mail.new Mail::Utilities.binary_unsafe_to_crlf(source.to_s)
   end

actionmailbox/lib/action_mailbox/mail_ext/recipients.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-module Mail
+module Mail # :nodoc: all
   class Message
     def recipients
       Array(to) + Array(cc) + Array(bcc) + Array(header[:x_original_to]).map(&:to_s) +