DOCS: add a changelog for option/command/property changes

All these make up both user interface and API. It's a good idea to
actually keep track of changes to it.

DOCS/client-api-changes.rst

@@ -4,9 +4,7 @@ Introduction
 This file lists all changes that can cause compatibility issues when using
 mpv through the client API (libmpv and ``client.h``). Since the client API
 interfaces to input handling (commands, properties) as well as command line
-options, this list is interesting for other uses of mpv, such as the Lua
-scripting interface, key bindings in ``input.rst``, or plain command line
-usage.
+options, you should also look at ``interface-changes.rst``.
 
 Normally, changes to the C API that are incompatible to previous iterations
 receive a major version bump (i.e. the first version number is increased),
@@ -20,6 +18,15 @@ The version number is the same as used for MPV_CLIENT_API_VERSION (see
 
 Also, read the section ``Compatibility`` in ``client.h``.
 
+Options, commands, properties
+=============================
+
+Changes to these are not listed here, but in ``interface-changes.rst``. (Before
+client API version 1.17, they were listed here partially.)
+
+This listing includes changes to the bare C API and behavior only, not what
+you can access with them.
+
 API changes
 ===========
 

DOCS/interface-changes.rst

@@ -0,0 +1,37 @@
+Introduction
+============
+
+mpv provides access to its internal via the following means:
+
+- options
+- commands
+- properties
+- events
+
+All of these are important for interfacing both with end users and API users
+(which include Lua scripts, libmpv, and the JSON IPC). As such, they constitute
+a large part of the user interface and APIs.
+
+This document lists changes to them. New changes are added to the top.
+
+Interface changes
+=================
+
+::
+
+ --- mpv 0.10.0 will be released ---
+    - change volume property:
+        - the value 100 is now always "unchanged volume" - with softvol, the
+          range is 0 to --softvol-max, without it is 0-100
+        - the minimum value of --softvol-max is raised to 100
+    - remove vo opengl npot suboption
+    - add relative seeking by percentage to "seek" command
+    - add playlist_shuffle command
+    - add --force-window=immediate
+    - add ao coreaudio change-physical-format suboption
+    - remove vo opengl icc-cache suboption, add icc-cache-dir suboption
+    - add --screenshot-directory
+    - add --screenshot-high-bit-depth
+    - add --screenshot-jpeg-source-chroma
+    - default action for "rescan_external_files" command changes
+ --- mpv 0.9.0 is released ---