[DOCS] improve explanation of dynamic mapping setting (#25829)

Closes #25825

docs/reference/mapping/params/dynamic.asciidoc

@@ -46,8 +46,11 @@ not.  It accepts three settings:
 
 [horizontal]
 `true`::    Newly detected fields are added to the mapping. (default)
-`false`::   Newly detected fields are ignored.  New fields must be added explicitly.
-`strict`::  If new fields are detected, an exception is thrown and the document is rejected.
+`false`::   Newly detected fields are ignored. These fields will not be indexed so will not be searchable 
+            but will still appear in the `_source` field of returned hits. These fields will not be added 
+            to the mapping,  new fields must be added explicitly.
+`strict`::  If new fields are detected, an exception is thrown and the document is rejected. New fields 
+            must be explicitly added to the mapping.
 
 The `dynamic` setting may be set at the mapping type level, and on each
 <<object,inner object>>.  Inner objects inherit the setting from their parent