Misleading docs for CsrMatrix::disassemble

The docs say:

> If the matrix contains the sole reference to the sparsity pattern, then the data is returned as-is. Otherwise, the sparsity pattern is cloned.

This stems from an earlier design in which the sparsity pattern was stored behind an `Arc`. In the current design, the sparsity pattern is always directly returned without any cloning (since it is not shared).