FireWire: add driver-api Introduction section

Replace the Introduction section's TBD with some useful overview text.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: linux-doc@vger.kernel.org
Cc: linux-scsi@vger.kernel.org
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/firewire.rst

@@ -5,17 +5,32 @@ Firewire (IEEE 1394) driver Interface Guide
 Introduction and Overview
 =========================
 
-TBD
+The Linux FireWire subsystem adds some interfaces into the Linux system to
+ use/maintain+any resource on IEEE 1394 bus.
+
+The main purpose of these interfaces is to access address space on each node
+on IEEE 1394 bus by ISO/IEC 13213 (IEEE 1212) procedure, and to control
+isochronous resources on the bus by IEEE 1394 procedure.
+
+Two types of interfaces are added, according to consumers of the interface. A
+set of userspace interfaces is available via `firewire character devices`. A set
+of kernel interfaces is available via exported symbols in `firewire-core` module.
 
 Firewire char device data structures
 ====================================
 
+.. include:: /ABI/stable/firewire-cdev
+    :literal:
+
 .. kernel-doc:: include/uapi/linux/firewire-cdev.h
     :internal:
 
 Firewire device probing and sysfs interfaces
 ============================================
 
+.. include:: /ABI/stable/sysfs-bus-firewire
+    :literal:
+
 .. kernel-doc:: drivers/firewire/core-device.c
     :export: