ICU-22534 add mf2 to docmain.h
diff --git a/icu4c/source/common/unicode/docmain.h b/icu4c/source/common/unicode/docmain.h
index 581b2e1..fa4dbbc 100644
--- a/icu4c/source/common/unicode/docmain.h
+++ b/icu4c/source/common/unicode/docmain.h
@@ -143,6 +143,11 @@
* <td>icu::MessageFormat</td>
* </tr>
* <tr>
+ * <td>Message Formatting 2<br/>(technology preview)</td>
+ * <td>(no C API)</td>
+ * <td>icu::message2::MessageFormatter</td>
+ * </tr>
+ * <tr>
* <td>List Formatting</td>
* <td>ulistformatter.h</td>
* <td>icu::ListFormatter</td>