8225562: jlink --module-path behavior inconsistent with help/errors

Reviewed-by: alanb

src/jdk.jlink/share/classes/jdk/tools/jlink/resources/jlink.properties

@@ -38,7 +38,12 @@ main.opt.version=\
 \      --version                         Version information
 
 main.opt.module-path=\
-\  -p, --module-path <path>              Module path
+\  -p, --module-path <path>              Module path.\n\
+\                                        If not specified, the JDK's jmods directory\n\
+\                                        will be used, if it exists. If specified,\n\
+\                                        but it does not contain the java.base module,\n\
+\                                        the JDK's jmods directory will be added,\n\
+\                                        if it exists.
 
 main.opt.add-modules=\
 \      --add-modules <mod>[,<mod>...]    Root modules to resolve
@@ -115,7 +120,7 @@ err.launcher.main.class.empty:launcher main class name cannot be empty: {0}
 err.launcher.module.name.empty:launcher module name cannot be empty: {0}
 err.launcher.value.format:launcher value should be of form <command>=<module>[/<main-class>]: {0}
 err.output.must.be.specified:--output must be specified
-err.modulepath.must.be.specified:--module-path must be specified
+err.modulepath.must.be.specified:--module-path is not specified and this runtime image does not contain jmods directory.
 err.mods.must.be.specified:no modules specified to {0}
 err.path.not.found=path not found: {0}
 err.path.not.valid=invalid path: {0}