docs: clarify Maven cache paths and key hash inputs (#1096)

Address post-merge review feedback on #1094: the intro paragraph
described the cache too narrowly. setup-java caches both
~/.m2/repository and ~/.m2/wrapper/dists, and the default key hashes
**/pom.xml plus .mvn/wrapper/maven-wrapper.properties and
.mvn/extensions.xml, so changing wrapper/extensions files also
invalidates the cache.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

docs/advanced-usage.md

@@ -289,9 +289,14 @@ To run the JavaFX application in CI:
 ## Ensuring the Maven cache is complete (plugin dependencies)
 
 When you enable `cache: maven`, the action caches your local Maven repository
-(`~/.m2/repository`) keyed on a hash of your `pom.xml` files, and — at the end of
-the job — saves whatever was downloaded during that run. It does **not** re-save
-the cache when the key already matches (a cache *hit*).
+(`~/.m2/repository`) and downloaded Maven Wrapper distributions
+(`~/.m2/wrapper/dists`). The cache key is a hash of your Maven inputs — every
+`**/pom.xml`, plus `**/.mvn/wrapper/maven-wrapper.properties` and
+`**/.mvn/extensions.xml` — so changing any of those files (for example bumping
+the wrapper version or editing core extensions) produces a new key and
+invalidates the cache. At the end of the job the action saves whatever was
+downloaded during that run. It does **not** re-save the cache when the key
+already matches (a cache *hit*).
 
 Maven resolves **plugin** dependencies lazily: it only downloads the plugins and
 plugin dependencies required by the goals that actually execute. As a result, the