[DOCS][101] Update first quick start with mappings examples

docs/reference/mapping.asciidoc

@@ -75,9 +75,23 @@ reindexing. You can use runtime fields in conjunction with indexed fields to
 balance resource usage and performance. Your index will be smaller, but with
 slower search performance.
 
+[discrete]
+[[mapping-combine-explicit-dynamic]]
+== Combine explicit and dynamic mappings
+
+Explicit mappings should almost always be defined at index creation, because
+you can't change mappings for fields that already contain data.
+
+You can _update_ mappings for existing fields under certain conditions:
+
+. You can add new fields to an existing mapping.
+. You can change the mapping for fields that don't contain any data.
+. You can add new <<multi-fields,multi-fields>> for existing fields.
+
 [discrete]
 [[mapping-limit-settings]]
-== Settings to prevent mapping explosion
+== Prevent mapping explosions
+
 Defining too many fields in an index can lead to a mapping explosion, which can
 cause out of memory errors and difficult situations to recover from.