Fix documentation for `--{,tool_}java_language_version`

Fixes: https://github.com/bazelbuild/bazel/issues/21317
PiperOrigin-RevId: 611061155
Change-Id: Id1c18c4e59a2fd896fd52214092b4d854a493bc8

toolchains/default_java_toolchain.bzl

diff --git a/toolchains/default_java_toolchain.bzl b/toolchains/default_java_toolchain.bzl
index d7dde59..78fdbc7 100644
--- a/toolchains/default_java_toolchain.bzl
+++ b/toolchains/default_java_toolchain.bzl
@@ -133,6 +133,8 @@
     header_compiler_direct = [Label("@remote_java_tools//:TurbineDirect")],
 )
 
+# If this is changed, the docs for "{,tool_}java_language_version" also
+# need to be updated in the Bazel user manual
 _DEFAULT_SOURCE_VERSION = "8"
 
 def default_java_toolchain(name, configuration = DEFAULT_TOOLCHAIN_CONFIGURATION, toolchain_definition = True, exec_compatible_with = [], target_compatible_with = [], **kwargs):