[DOCS] _index_prefix for highligh matched_fields

Enhance documenation to explain that "_index_prefix" subfield must
be added to `matched_fields` param for highlighting a main field.
When doing prefix queries on fields that are indexed with prefixes,
"_index_prefix" subfield is used. If we try to highlight the main
field, we may not get any results. "_index_prefix" subfield must
be added to `matched_fields` which instructs ES to use matches
from "_index_prefix" to highlight the main field.

docs/reference/mapping/params/index-prefixes.asciidoc

@@ -54,3 +54,29 @@ PUT my-index-000001
   }
 }
 --------------------------------
+
+`index_prefixes` parameter instructs {ES} to create a subfield "._index_prefix". This
+field will be used to do fast prefix queries. When doing highlighting, add "._index_prefix"
+subfield to the `matched_fields` parameter to highlight the main field based on the
+found matches of the prefix field, like in the request below:
+
+[source,console]
+--------------------------------
+GET my-index-000001/_search
+{
+  "query": {
+    "prefix": {
+      "full_name": {
+        "value": "ki"
+      }
+    }
+  },
+  "highlight": {
+    "fields": {
+      "full_name": {
+        "matched_fields": ["full_name._index_prefix"]
+      }
+    }
+  }
+}
+--------------------------------

docs/reference/mapping/types/search-as-you-type.asciidoc

@@ -97,11 +97,21 @@ GET my-index-000001/_search
         "my_field._3gram"
       ]
     }
+  },
+  "highlight": {
+    "fields": {
+      "my_field": {
+        "matched_fields": ["my_field._index_prefix"] <1>
+      }
+    }
   }
 }
 --------------------------------------------------
 // TEST[continued]
 
+<1> Adding "my_field._index_prefix" to the `matched_fields` allows to highlight
+    "my_field" also based on matches from "my_field._index_prefix" field.
+
 [source,console-result]
 --------------------------------------------------
 {
@@ -126,6 +136,11 @@ GET my-index-000001/_search
         "_score" : 0.8630463,
         "_source" : {
           "my_field" : "quick brown fox jump lazy dog"
+        },
+        "highlight": {
+          "my_field": [
+            "quick <em>brown fox jump lazy</em> dog"
+          ]
         }
       }
     ]