Function unnest_array 

pub fn unnest_array(
    doc: &Document,
    path: &[PathSegment],
) -> StorageBackendResult<Vec<Document>>

Unnest an array at path into a sequence of synthesised documents. Each emitted document is a clone of the source merged with two metadata fields:

• <path>._unnested – the array element value.
• _unnest_index – the element’s 0-based index.