📝 fix docs (#5217)

docs/mkdocs/docs/api/basic_json/operator=.md

@@ -5,7 +5,8 @@ basic_json& operator=(basic_json other) noexcept (
     std::is_nothrow_move_constructible<value_t>::value &&
     std::is_nothrow_move_assignable<value_t>::value &&
     std::is_nothrow_move_constructible<json_value>::value &&
-    std::is_nothrow_move_assignable<json_value>::value
+    std::is_nothrow_move_assignable<json_value>::value &&
+    std::is_nothrow_move_assignable<json_base_class_t>::value
 );
 ```
 
@@ -17,6 +18,10 @@ constructor, destructor, and the `swap()` member function.
 `other` (in)
 :   value to copy from
 
+## Exception safety
+
+Strong guarantee: if an exception is thrown while copying `other`, there are no changes to `#!cpp *this`.
+
 ## Complexity
 
 Linear.
@@ -38,6 +43,11 @@ Linear.
     --8<-- "examples/basic_json__copyassignment.output"
     ```
 
+## See also
+
+- [basic_json](basic_json.md) create a JSON value
+- [swap](swap.md) exchanges the contents of two JSON values
+
 ## Version history
 
 - Added in version 1.0.0.