docs: rapidio: add it to the driver API

This is actually a subsystem description, with contains both
kAPI and uAPI.

While it should ideally be slplit, let's place it at driver-api,
as most things are related to kAPI and driver-specific info.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/admin-guide/index.rst

@@ -61,6 +61,7 @@ configure specific aspects of kernel behavior to your liking.
    parport
    md
    module-signing
+   rapidio
    sysrq
    unicode
    vga-softcursor

Documentation/driver-api/index.rst

@@ -45,7 +45,7 @@ available subsections can be seen below.
    miscellaneous
    mei/index
    w1
-   rapidio
+   rapidio/index
    s390-drivers
    vme
    80211/index

Documentation/rapidio/index.rst → Documentation/driver-api/rapidio/index.rst

-:orphan:
-
 ===========================
 The Linux RapidIO Subsystem
 ===========================

drivers/rapidio/Kconfig

@@ -86,7 +86,7 @@ config RAPIDIO_CHMAN
 	  This option includes RapidIO channelized messaging driver which
 	  provides socket-like interface to allow sharing of single RapidIO
 	  messaging mailbox between multiple user-space applications.
-	  See "Documentation/rapidio/rio_cm.rst" for driver description.
+	  See "Documentation/driver-api/rapidio/rio_cm.rst" for driver description.
 
 config RAPIDIO_MPORT_CDEV
 	tristate "RapidIO /dev mport device driver"