Add note to put mapping API docs on index creation

This commit clarifies that the put mapping API can also apply when
creating an index.

Closes #19703

docs/reference/indices/put-mapping.asciidoc

@@ -1,7 +1,7 @@
 [[indices-put-mapping]]
 == Put Mapping
 
-The PUT mapping API allows you to add a new type to an existing index, or new
+The PUT mapping API allows you to provide type mappings while creating a new index, add a new type to an existing index, or add new
 fields to an existing type:
 
 [source,js]