Migrate the BMailComponent docs to the Haiku Book.

I've updated the docs to match the current BMailComponent, documented
new functions, and cleaned up the MailComponent.h file to at least
somewhat match our coding style.

First in a series (there are 3 more old API docs on the Mail Kit in that
"Public API" folder.)

docs/user/book.dox

@@ -43,6 +43,8 @@
 		- The \ref locale includes classes to localize your application to
 			different languages, timezones, number formatting conventions and
 			much more.
+		- The \ref mail includes classes to work with e-mail files, folders,
+			protocols, and filters, as part of Haiku's unique mail handling system.
 		- The \ref media provides a unified and consistent interface for media
 			streams and applications to intercommunicate.
 		- The \ref midi2 describes an interface to generating, processing,
@@ -135,6 +137,9 @@
 	\defgroup locale Locale Kit
 	\brief Collection of classes for localizing applications.
 
+	\defgroup mail Mail Kit
+	\brief API for working with e-mail messages and protocols.
+
 	\defgroup media Media Kit
 	\brief Collection of classes that deal with audio and video.
 
@@ -453,9 +458,9 @@ snooze_until(time - Latency(), B_SYSTEM_TIMEBASE);
 
 	The Haiku Network Kit consists of:
 	- A modular, add-ons based network stack
-	- Two shared libraries, libnet.so and libnetapi.so
+	- Two shared libraries, libnetwork.so and libnetapi.so
 	- A stack driver, acting as interface between the network stack and
-	  libnet.so
+	  libnetwork.so
 	- Basic network apps
 	- A modular GUI preflet