It’s mentioned in the documentation (and in the community forum). Refer to the description of an entity’s state property and to what is returned by the states() function.
In addition, an entity’s state property can only store a string whose maximum length is 255 characters (it will truncate anything longer).
In contrast, an entity’s attributes can store values whose type is string, int, float, dict, list, boolean, etc up to about 16 Kb.
EDIT
Admittedly, those last two characteristics might not exist in the documentation.