docs: add section titles

kenjis · kenjis · commit b2d317d15c17 · 2023-04-12T14:08:15.000+09:00
diff --git a/user_guide_src/source/libraries/validation.rst b/user_guide_src/source/libraries/validation.rst
@@ -370,10 +370,16 @@ rules. As shown earlier, the validation array will have this prototype:
 
 .. literalinclude:: validation/013.php
 
+How to Specify Rule Group
+-------------------------
+
 You can specify the group to use when you call the ``run()`` method:
 
 .. literalinclude:: validation/014.php
 
+How to Save Error Messages
+--------------------------
+
 You can also store custom error messages in this configuration file by naming the
 property the same as the group, and appended with ``_errors``. These will automatically
 be used for any errors when this group is used:
