media: ca-reset.rst: add some description to this ioctl

While we don't have any documentation for it, based on what's
there at Kaffeine and VDR, it seems that this command should
be issued before start using CA. So, document it as such.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/dvb/ca-reset.rst

@@ -28,7 +28,8 @@ Arguments
 Description
 -----------
 
-.. note:: This ioctl is undocumented. Documentation is welcome.
+Puts the Conditional Access hardware on its initial state. It should
+be called before start using the CA hardware.
 
 
 Return Value