Misleading docs for CsrMatrix::disassemble

[Andlon]

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).