feat(java): document multiple source roots feature of Maven plugin (#13285)

Author: lcian

docs/platforms/java/common/maven/index.mdx

@@ -90,3 +90,13 @@ The plugin automatically adds the Sentry Java SDK as well as available Sentry in
 ## Source Context
 
 See our documentation on <PlatformLink to="/source-context/">Source Context</PlatformLink>.
+
+By default, all of the source roots that are included as part of your build are sent to Sentry for source context.
+
+To manually specify additional directories that shall be included in the source bundle, use the following configuration within your `plugin` tag for `io.sentry:sentry-maven-plugin`:
+```xml
+<additionalSourceDirsForSourceContext>
+  <value>src/main/someDirectory</value>
+  <value>src/main/someOtherDirectory</value>
+</additionalSourceDirsForSourceContext>
+```