Clarify impact of java version used on /google_java_format README.

Manually created Google internal equivalent of public #1228, just in case it's easier to review here instead of there; quote with rationale for this, from public PR description:

It at first confused the hell out of me why clearly the exact SAME version of google-java-format worked great locally but failed with errors (due to my use of record matching pattern syntax) on a GitHub Action - until I've noticed that it still ran under Java 17 - and that this matters to google-java-format (my naive initial assumption was that only the google-java-format version itself was relevant).

PiperOrigin-RevId: 741076633

Author: vorburger

README.md

@@ -16,6 +16,11 @@ and run it with:
 java -jar /path/to/google-java-format-${GJF_VERSION?}-all-deps.jar <options> [files...]
 ```
 
+Note that it uses the `jdk.compiler` module to parse the Java source code. The
+`java` binary version used must therefore be from a JDK (not JRE) with a version
+equal to or newer than the Java language version of the files being formatted.
+An alternative is to use the available GraalVM based native binaries instead.
+
 The formatter can act on whole files, on limited lines (`--lines`), on specific
 offsets (`--offset`), passing through to standard-out (default) or altered
 in-place (`--replace`).