Update reference docs for Highlighter fragmenter (#23754)

Explain the fragmenter and add examples.

docs/reference/search/request/highlighting.asciidoc

@@ -401,6 +401,128 @@ GET /_search
 // CONSOLE
 // TEST[setup:twitter]
 
+==== Fragmenter
+
+Fragmenter can control how text should be broken up in highlight snippets.
+However, this option is applicable only for the Plain Highlighter.
+There are two options:
+
+[horizontal]
+`simple`:: Breaks up text into same sized fragments.
+`span`:: Same as the simple fragmenter, but tries not to break up text between highlighted terms (this is applicable when using phrase like queries). This is the default.
+
+[source,js]
+--------------------------------------------------
+GET twitter/tweet/_search
+{
+    "query" : {
+        "match_phrase": { "message": "number 1" }
+    },
+    "highlight" : {
+        "fields" : {
+            "message" : {
+                "fragment_size" : 15,
+                "number_of_fragments" : 3,
+                "fragmenter": "simple"
+            }
+        }
+    }
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[setup:twitter]
+
+Response:
+
+[source,js]
+--------------------------------------------------
+{
+    ...
+    "hits": {
+        "total": 1,
+        "max_score": 1.4818809,
+        "hits": [
+            {
+                "_index": "twitter",
+                "_type": "tweet",
+                "_id": "1",
+                "_score": 1.4818809,
+                "_source": {
+                    "user": "test",
+                    "message": "some message with the number 1",
+                    "date": "2009-11-15T14:12:12",
+                    "likes": 1
+                },
+                "highlight": {
+                    "message": [
+                        " with the <em>number</em>",
+                        " <em>1</em>"
+                    ]
+                }
+            }
+        ]
+    }
+}
+--------------------------------------------------
+// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,/]
+
+[source,js]
+--------------------------------------------------
+GET twitter/tweet/_search
+{
+    "query" : {
+        "match_phrase": { "message": "number 1" }
+    },
+    "highlight" : {
+        "fields" : {
+            "message" : {
+                "fragment_size" : 15,
+                "number_of_fragments" : 3,
+                "fragmenter": "span"
+            }
+        }
+    }
+}
+--------------------------------------------------
+// CONSOLE
+// TEST[setup:twitter]
+
+Response:
+
+[source,js]
+--------------------------------------------------
+{
+    ...
+    "hits": {
+        "total": 1,
+        "max_score": 1.4818809,
+        "hits": [
+            {
+                "_index": "twitter",
+                "_type": "tweet",
+                "_id": "1",
+                "_score": 1.4818809,
+                "_source": {
+                    "user": "test",
+                    "message": "some message with the number 1",
+                    "date": "2009-11-15T14:12:12",
+                    "likes": 1
+                },
+                "highlight": {
+                    "message": [
+                        "some message with the <em>number</em> <em>1</em>"
+                    ]
+                }
+            }
+        ]
+    }
+}
+--------------------------------------------------
+// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,/]
+
+If the `number_of_fragments` option is set to `0`,
+`NullFragmenter` is used which does not fragment the text at all.
+This is useful for highlighting the entire content of a document or field.
 
 ==== Highlight query