Show how to enable the IntelliJ plugin, format on save and use the native binary

Two screenshots: the open-java-format Settings page with the plugin enabled,
and Actions on Save with Reformat code for Java files and the whole file, the
scope the GitHub Action checks. A new section explains the native binary in
the IDE: it is switched on by openjavaformat.native.formatter in the Gradle
build, and the settings page then shows Native image formatter.

Author: abashev

docs/assets/images/intellij-actions-on-save.webp

@@ -35,17 +35,49 @@ A version that is not on the Marketplace yet can be installed from a release.
 
 The plugin is off in every project until you turn it on.
 
-1. Open **Settings** and search for **open-java-format Settings**.
+1. Open **Settings** and go to **Other Settings → open-java-format Settings**.
 2. Tick **Enable open-java-format**.
 
+![The open-java-format Settings page with Enable open-java-format ticked](../assets/images/intellij-enable.webp){ width="1000" height="673" loading="lazy" }
+
 From then on Reformat Code, ++ctrl+alt+l++ or ++option+cmd+l++ on macOS, formats Java files with
 open-java-format.
 
-## Which formatter version runs
+## Format on save
+
+1. Open **Settings** and go to **Tools → Actions on Save**.
+2. Tick **Reformat code**.
+3. Next to it, choose **Java** in the file list and **Whole file** instead of **Changed lines**.
 
-The formatter runs in a process of its own, on the IDE's runtime, so the project SDK can be any
-version. When a Gradle build runs the formatter as a
-[native binary](gradle.md#choose-how-the-formatter-runs), the IDE runs that binary instead.
+![Actions on Save with Reformat code ticked for Java files and the whole file](../assets/images/intellij-actions-on-save.webp){ width="1000" height="674" loading="lazy" }
+
+Limiting it to Java keeps other files out: the IDE would reformat those with its own formatters.
+**Whole file** formats the entire file, which is what the [GitHub Action](github-actions.md) checks.
+**Changed lines** formats only the lines you edited, the way `formatDiff` does.
+
+## Which formatter version runs
 
 A project that applies the [Gradle plugin](gradle.md) takes the formatter version from the build.
-Every other project uses the version bundled with the IDE plugin.
+Every other project uses the version bundled with the IDE plugin, which the settings page shows as
+`(bundled)` under **Implementation version**, as in the screenshot above.
+
+By default the formatter runs in a process of its own, on the IDE's runtime, so the project SDK can
+be any version.
+
+## Native binary
+
+In a project with the [Gradle plugin](gradle.md), the IDE can run the formatter as the native binary
+that the build uses. It is switched on in the build, not in the IDE:
+
+``` properties title="gradle.properties"
+openjavaformat.native.formatter=true
+```
+
+After the next Gradle sync in the IDE, the plugin writes the path of the binary into
+`.idea/open-java-format.xml`, and **Formatter type** on the settings page changes from
+**Java-based formatter** to **Native image formatter**. Each reformat starts the formatter as a new
+process, and the native binary starts without a JVM.
+
+The binary runs on the platforms listed on the
+[Gradle plugin](gradle.md#choose-how-the-formatter-runs) page. Elsewhere the plugin ignores the
+property, and the IDE keeps the Java-based formatter.