Docs: Update highlighting.asciidoc

Added note about how to highlight on the `_all` field

Closes #7991

docs/reference/search/request/highlighting.asciidoc

@@ -23,11 +23,17 @@ search hit (there will be another element in each search hit, called
 `highlight`, which includes the highlighted fields and the highlighted
 fragments).
 
+[NOTE]
+==================================
 In order to perform highlighting, the actual content of the field is
 required. If the field in question is stored (has `store` set to `true`
 in the mapping) it will be used, otherwise, the actual `_source` will
 be loaded and the relevant field will be extracted from it.
 
+The `_all` field cannot be extracted from `_source`, so it can only
+be used for highlighting if it mapped to have `store` set to `true`.
+==================================
+
 The field name supports wildcard notation. For example, using `comment_*`
 will cause all fields that match the expression to be highlighted.