Updated the field_caps docs to describe family types

Author: markharwood

docs/reference/search/field-caps.asciidoc

@@ -67,6 +67,13 @@ query rewrites to `match_none` on every shard.
 ==== {api-response-body-title}
 
 
+The types used in the response describe _families_ of field types.
+Normally a family type is the same as the field type declared in the mapping,
+but to simplify matters certain field types that behave identically are
+described using a family type. For example, `keyword`, `constant_keyword` and `wildcard`
+field types are all described as the `keyword` family type.
+
+
 
 `searchable`::
   Whether this field is indexed for search on all indices.
@@ -75,8 +82,8 @@ query rewrites to `match_none` on every shard.
   Whether this field can be aggregated on all indices.
 
 `indices`::
-  The list of indices where this field has the same type, or null if all indices
-  have the same type for the field.
+  The list of indices where this field has the same family type, or null if all indices
+  have the same family type for the field.
 
 `non_searchable_indices`::
   The list of indices where this field is not searchable, or null if all indices