Add documentation about version constraint to validation docs (#10012)

* Add documentation about version constraint to validation docs

* Update validation.md

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: shaedrich

validation.md

@@ -1887,6 +1887,12 @@ The field under validation must be a valid [Universally Unique Lexicographically
 
 The field under validation must be a valid RFC 4122 (version 1, 3, 4, or 5) universally unique identifier (UUID).
 
+You may also validate that the given UUID matches a UUID specification by version:
+
+```php
+'uuid' => 'uuid:4'
+```
+
 <a name="conditionally-adding-rules"></a>
 ## Conditionally Adding Rules