Docs: synthetic _source can remove some arrays (#91632)

Synthetic _source's array flattening activities can remove some arrays
entirely. Specifically:
```
{
  "foo": [
    {
      "bar": 1
    },
    {
      "baz": 2
    }
  ]
}
```

Turns into:
```
{
  "foo": {
    "bar": 1,
    "baz": 2
  }
}
```

See, no more array! It's because the values are flattend to the leaf
fields and didn't have multiple values. This is implied by the docs we
had, but sure wasn't obvious. So now it's documented specifically.

docs/reference/mapping/fields/synthetic-source.asciidoc

@@ -89,6 +89,37 @@ Will become:
 ----
 // TEST[s/^/{"_source":/ s/\n$/}/]
 
+This can cause some arrays to vanish:
+
+[source,console,id=synthetic-source-leaf-arrays-example-sneaky]
+----
+PUT idx/_doc/1
+{
+  "foo": [
+    {
+      "bar": 1
+    },
+    {
+      "baz": 2
+    }
+  ]
+}
+----
+// TEST[s/$/\nGET idx\/_doc\/1?filter_path=_source\n/]
+
+Will become:
+
+[source,console-result]
+----
+{
+  "foo": {
+    "bar": 1,
+    "baz": 2
+  }
+}
+----
+// TEST[s/^/{"_source":/ s/\n$/}/]
+
 [[synthetic-source-modifications-field-names]]
 ====== Fields named as they are mapped
 Synthetic source names fields as they are named in the mapping. When used