media: docs: move fourcc file to the uAPI document

This file gives a hint about how fourcc should be named.

It is on a very weird place, as such kind of thing belongs to the
uAPI guide.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/media/v4l-drivers/index.rst

@@ -31,7 +31,6 @@ For more details see the file COPYING in the source distribution of Linux.
 	:maxdepth: 5
 	:numbered:
 
-	fourcc
 	v4l-with-ir
 	tuners
 	cardlist

Documentation/userspace-api/media/v4l/common.rst

@@ -57,6 +57,7 @@ applicable to all devices.
     ext-ctrls-fm-tx
     ext-ctrls-fm-rx
     ext-ctrls-detect
+    fourcc
     format
     planar-apis
     selection-api