FireWire: add driver-api Introduction section

Replace the Introduction section's TBD with some useful overview text.

Signed-off-by: Takashi Sakamoto <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Stefan Richter <[e-mail]>
Cc: Takashi Sakamoto <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Randy Dunlap <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: "James E.J. Bottomley" <[e-mail]>
Cc: "Martin K. Petersen" <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/driver-api/firewire.rst

···
 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: