Mark exists-query dsl doc properly

All the docs for the `exists` query that aren't marked as `CONSOLE`
aren't actually `CONSOLE`-worthy so this marks them as `NOTCONSOLE`.

It also rewrites the text around `missing` query. Since it was
removed in 5.0 we don't need to talk about it in the 6.0 docs.

Relates to #18160

docs/build.gradle

@@ -107,7 +107,6 @@ buildRestTests.expectedUnconvertedCandidates = [
   'reference/modules/scripting/security.asciidoc',
   'reference/modules/scripting/using.asciidoc',
   'reference/modules/cross-cluster-search.asciidoc', // this is hard to test since we need 2 clusters -- maybe we can trick it into referencing itself...
-  'reference/query-dsl/exists-query.asciidoc',
   'reference/query-dsl/function-score-query.asciidoc',
   'reference/query-dsl/geo-shape-query.asciidoc',
   'reference/search/field-stats.asciidoc',

docs/reference/query-dsl/exists-query.asciidoc

@@ -24,6 +24,7 @@ For instance, these documents would all match the above query:
 { "user": ["jane"] }
 { "user": ["jane", null ] } <3>
 --------------------------------------------------
+// NOTCONSOLE
 <1> An empty string is a non-`null` value.
 <2> Even though the `standard` analyzer would emit zero tokens, the original field is non-`null`.
 <3> At least one non-`null` value is required.
@@ -37,6 +38,7 @@ These documents would *not* match the above query:
 { "user": [null] } <2>
 { "foo":  "bar" } <3>
 --------------------------------------------------
+// NOTCONSOLE
 <1> This field has no values.
 <2> At least one non-`null` value is required.
 <3> The `user` field is missing completely.
@@ -50,11 +52,21 @@ instance, if the `user` field were mapped as follows:
 
 [source,js]
 --------------------------------------------------
-  "user": {
-    "type": "text",
-    "null_value": "_null_"
+PUT /example
+{
+  "mapping": {
+    "doc": {
+      "properties": {
+        "user": {
+          "type": "text",
+          "null_value": "_null_"
+        }
+      }
+    }
   }
+}
 --------------------------------------------------
+// CONSOLE
 
 then explicit `null` values would be indexed as the string `_null_`, and the
 following docs would match the `exists` filter:
@@ -64,6 +76,7 @@ following docs would match the `exists` filter:
 { "user": null }
 { "user": [null] }
 --------------------------------------------------
+// NOTCONSOLE
 
 However, these docs--without explicit `null` values--would still have
 no values in the `user` field and thus would not match the `exists` filter:
@@ -73,11 +86,12 @@ no values in the `user` field and thus would not match the `exists` filter:
 { "user": [] }
 { "foo": "bar" }
 --------------------------------------------------
+// NOTCONSOLE
 
 ==== `missing` query
 
-'missing' query has been removed because it can be advantageously replaced by an `exists` query inside a must_not
-clause as follows:
+There isn't a `missing` query. Instead use the `exists` query inside a
+`must_not` clause as follows:
 
 [source,js]
 --------------------------------------------------
@@ -97,4 +111,3 @@ GET /_search
 // CONSOLE
 
 This query returns documents that have no value in the user field.
-