Clarify dependency override instructions for Thymeleaf 3

Closes gh-7097

Author: snicoll

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -1457,9 +1457,9 @@ have been applied from the auto-configuration:
 
 [[howto-use-thymeleaf-3]]
 === Use Thymeleaf 3
-By default, `spring-boot-starter-thymeleaf` uses Thymeleaf 2.1. You can use Thymeleaf
-3 by overriding the `thymeleaf.version` and `thymeleaf-layout-dialect.version` properties,
-for example:
+By default, `spring-boot-starter-thymeleaf` uses Thymeleaf 2.1. If you are using the
+`spring-boot-starter-parent`, you can use Thymeleaf 3 by overriding the
+`thymeleaf.version` and `thymeleaf-layout-dialect.version` properties, for example:
 
 [source,xml,indent=0,subs="verbatim,quotes,attributes"]
 ----
@@ -1469,6 +1469,9 @@ for example:
 	</dependency>
 ----
 
+NOTE: if you are managing dependencies yourself, look at `spring-boot-dependencies` for
+the list of artifacts that are related to those two versions.
+
 To avoid a warning message about the HTML 5 template mode being deprecated and the HTML
 template mode being used instead, you may also want to explicitly configure
 `spring.thymeleaf.mode` to be `HTML`, for example: