Annotation used to mark a POJO property to be automatically populated with the document's ID when the POJO is created from a Cloud Firestore document (for example, via toObject(Class)).

Any of the following will throw a runtime exception:
• This annotation is applied to a property of a type other than String or DocumentReference.
• This annotation is applied to a property that is not writable (for example, a Java Bean getter without a backing field).
• This annotation is applied to a property with a name that conflicts with a read document field. For example, if a POJO has a field `firstName` annotated by @DocumentId, and there is a property from the document named `firstName` as well, an exception is thrown when you try to read the document into the POJO via toObject(Class) or get().

When using a POJO to write to a document (via set(Object) or @set(DocumentReference, Object)), the property annotated by @DocumentId is ignored, which allows writing the POJO back to any document, even if it's not the origin of the POJO.