Update migration guide for 3.0

Author: danburzo

docs/api.md

@@ -1599,12 +1599,9 @@ import { useParser } from 'culori';
 
 // Register custom parser
 useParser(function(str) => {
-  return {
-    mode: 'rgb',
-    r: 0,
-    g: 0,
-    b: 0
-  };
+  let color = {};
+  // parse the string
+  return color;
 });
 
 // Register `color(--oklab)` syntax
@@ -1615,7 +1612,7 @@ useParser('--oklab', 'oklab');
 
 <span aria-label='Source:'>☞</span> [src/modes.js]({{codebase}}/src/modes.js)
 
-Remove a previously registered parser function.
+Remove a previously registered parser function or string, including parsers registered by default.
 
 ```js
 import { parse, parseNamed, removeParser } from 'culori';

docs/guides/migration.md

@@ -4,7 +4,25 @@ title: 'Migration guide'
 
 ## Migrating from version 2.x to 3.0
 
-### 
+For better alignment with the CSS Color Level 4 specification, support for the following syntaxes has changed:
+
+* `color(--oklab)` removed in favor of `oklab()`
+* `color(--oklch)` removed in favor of `oklch()`
+* `color(--srgb-linear)` removed in favor of `color(srgb-linear)`
+* `color(--xyz-d50)` removed in favor of `color(xyz-d50)`
+* `color(--xyz-d65)` removed in favor of `color(xyz)` and `color(xyz-d65)`
+
+The custom identifiers removed in this release can be re-registered with [`useParser()`](/api/#useParser), using string arguments:
+
+```js
+import { useParser } from 'culori';
+
+useParser('--oklab', 'oklab');
+useParser('--oklch', 'oklch');
+useParser('--srgb-linear', 'lrgb');
+useParser('--xyz-d50', 'xyz50');
+useParser('--xyz-d65', 'xyz65');
+```
 
 ## Migrating from version 1.x to 2.0