Add a link to the locale docs in the locale warning

Author: bjfish

src/main/java/org/truffleruby/core/encoding/EncodingManager.java

@@ -173,11 +173,12 @@ private void initializeLocaleEncoding(TruffleNFIPlatform nfi, NativeConfiguratio
                 // The parent process seems to explicitly want a C locale (e.g. EnvUtil#invoke_ruby in the MRI test harness), so only warn at config level in this case.
                 RubyLanguage.LOGGER.config(
                         "Encoding.find('locale') is US-ASCII, this often indicates that the system locale is not set properly. " +
-                                "Warning at level=CONFIG because LANG=C and LC_ALL=C are set.");
+                                "Warning at level=CONFIG because LANG=C and LC_ALL=C are set. " +
+                                "Set LANG=en_US.UTF-8 and see https://www.graalvm.org/dev/reference-manual/ruby/UTF8Locale/ for details.");
             } else {
                 RubyLanguage.LOGGER.warning(
                         "Encoding.find('locale') is US-ASCII, this often indicates that the system locale is not set properly. " +
-                                "Set LANG=C and LC_ALL=C to suppress this warning (but some things might break).");
+                                "Set LANG=en_US.UTF-8 and see https://www.graalvm.org/dev/reference-manual/ruby/UTF8Locale/ for details.");
             }
         }