media: ca.h: document ca_msg and the corresponding ioctls

Usually, CA messages are sent/received via reading/writing at
the CA device node. However, two drivers (dst_ca and firedtv-ci)
also implement it via ioctls.

Apparently, on both cases, the net result is the same.

Anyway, let's document it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/dvb/ca-get-msg.rst

@@ -28,22 +28,15 @@ Arguments
 ``msg``
   Pointer to struct :c:type:`ca_msg`.
 
-.. c:type:: ca_msg
-
-.. code-block:: c
-
-    /* a message to/from a CI-CAM */
-    struct ca_msg {
-	unsigned int index;
-	unsigned int type;
-	unsigned int length;
-	unsigned char msg[256];
-    };
-
 Description
 -----------
 
-.. note:: This ioctl is undocumented. Documentation is welcome.
+Receives a message via a CI CA module.
+
+.. note::
+
+   Please notice that, on most drivers, this is done by reading from
+   the /dev/adapter?/ca? device node.
 
 
 Return Value

Documentation/media/uapi/dvb/ca-send-msg.rst

@@ -32,8 +32,12 @@ Arguments
 Description
 -----------
 
-.. note:: This ioctl is undocumented. Documentation is welcome.
+Sends a message via a CI CA module.
 
+.. note::
+
+   Please notice that, on most drivers, this is done by writing
+   to the /dev/adapter?/ca? device node.
 
 Return Value
 ------------

include/uapi/linux/dvb/ca.h

@@ -101,7 +101,16 @@ struct ca_caps {
 	unsigned int descr_type;
 };
 
-/* a message to/from a CI-CAM */
+/**
+ * struct ca_msg - a message to/from a CI-CAM
+ *
+ * @index:	unused
+ * @type:	unused
+ * @length:	length of the message
+ * @msg:	message
+ *
+ * This struct carries a message to be send/received from a CI CA module.
+ */
 struct ca_msg {
 	unsigned int index;
 	unsigned int type;