DOC Add elemental block validation to 5.3.0 changelog

en/08_Changelogs/5.3.0.md

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+---
+title: 5.3.0 (unreleased)
+---
+
+# 5.3.0 (unreleased)
+
+## Overview
+
+- [Features and enhancements](#features-and-enhancements)
+  - [Validation for elemental content blocks when saving individual blocks](#elemental-validation)
+- [Bug fixes](#bug-fixes)
+
+## Features and enhancements
+
+### Validation for elemental content blocks when saving individual blocks {#elemental-validation}
+
+Elemental content blocks now supports validation when saving or publishing individual content blocks using the "three-dot" context menu in the top-right of the block.
+
+Validation can be added to a content block using standard [Model Validation and Constraints](https://docs.silverstripe.org/en/5/developer_guides/model/validation/#validation-and-constraints) by implementing a `validate()` method on a content block, or by using [Form Validation](https://docs.silverstripe.org/en/5/developer_guides/forms/validation/#form-validation) where there are several options available.
+
+## Bug fixes
+
+This release includes a number of bug fixes to improve a broad range of areas. Check the change logs for full details of these fixes split by module. Thank you to the community members that helped contribute these fixes as part of the release!
+
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