Overwork documentation (#4516)

docs/mkdocs/docs/api/basic_json/operator[].md

@@ -34,6 +34,15 @@ const_reference operator[](const json_pointer& ptr) const;
     [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
     This can also be a string view (C++17).
 
+## Iterator invalidation
+
+For the non-const versions 1. and 4., when passing an **array** index that does not exist, it is created and filled with
+a `#!json null` value before a reference to it is returned. For this, a reallocation can happen, in which case all
+iterators (including the [`end()`](end.md) iterator) and all references to the elements are invalidated.
+
+For [`ordered_json`](../ordered_json.md), also passing an **object key** to the non-const versions 2., 3., and 4., a
+reallocation can happen which again invalidates all iterators and all references.
+
 ## Parameters
 
 `idx` (in)