add removed custom separator support to breaking changes in v3 changelog

Author: dcastil

docs/changelog/v3-changelog.md

@@ -13,6 +13,7 @@ Check out the [migration guide](./v2-to-v3-migration.md) and if you have any que
 - `isLength` validator was removed and split into separate validators `isNumber` and `isFraction` by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
 - Prefix defined in config shouldn't include combining `-` character anymore by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
 - Tailwind CSS v3 prefix position in class not supported anymore in favor of Tailwind CSS v4 position by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
+- Custom separators are no longer supported by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
 - New mandatory `orderSensitiveModifiers` property in config when using `createTailwindMerge`, etc. by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
 - `DefaultThemeGroupIds` type union consists of different string literals than before by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)
 - Classes removed in Tailwind CSS v4 are not supported by tailwind-merge anymore by [@dcastil](https://github.com/dcastil) in [#518](https://github.com/dcastil/tailwind-merge/pull/518)