media: docs: clarify relationship between crop and selection APIs

Having two somewhat similar and largely overlapping APIs is confusing,
especially since the older one appears in the docs before the newer
and most featureful counterpart.

Clarify all of this in several ways:
 - swap the two sections
 - give a name to the two APIs in the section names
 - add a note at the beginning of the CROP API section
 - update note about VIDIOC_CROPCAP

Also remove a note that is incorrect (correct wording is in
vidioc-cropcap.rst).

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/uapi/v4l/common.rst

@@ -41,6 +41,6 @@ applicable to all devices.
     extended-controls
     format
     planar-apis
-    crop
     selection-api
+    crop
     streaming-par

Documentation/media/uapi/v4l/crop.rst

@@ -2,9 +2,18 @@
 
 .. _crop:
 
-*************************************
-Image Cropping, Insertion and Scaling
-*************************************
+*****************************************************
+Image Cropping, Insertion and Scaling -- the CROP API
+*****************************************************
+
+.. note::
+
+   The CROP API is mostly superseded by the newer :ref:`SELECTION API
+   <selection-api>`. The new API should be preferred in most cases,
+   with the exception of pixel aspect ratio detection, which is
+   implemented by :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` and has no
+   equivalent in the SELECTION API. See :ref:`selection-vs-crop` for a
+   comparison of the two APIs.
 
 Some video capture devices can sample a subsection of the picture and
 shrink or enlarge it to an image of arbitrary size. We call these
@@ -42,10 +51,9 @@ where applicable) will be fixed in this case.
 
 .. note::
 
-   All capture and output devices must support the
-   :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>` ioctl such that applications
-   can determine if scaling takes place.
-
+   All capture and output devices that support the CROP or SELECTION
+   API will also support the :ref:`VIDIOC_CROPCAP <VIDIOC_CROPCAP>`
+   ioctl.
 
 Cropping Structures
 ===================

Documentation/media/uapi/v4l/selection-api-005.rst

@@ -1,5 +1,7 @@
 .. -*- coding: utf-8; mode: rst -*-
 
+.. _selection-vs-crop:
+
 ********************************
 Comparison with old cropping API
 ********************************

Documentation/media/uapi/v4l/selection-api.rst

@@ -2,8 +2,8 @@
 
 .. _selection-api:
 
-API for cropping, composing and scaling
-=======================================
+Cropping, composing and scaling -- the SELECTION API
+====================================================
 
 
 .. toctree::