Update sparse crate docs

[SeabertYuan]

A quick PR addressing some documentation issues in the nalgebra-sparse crate:

• outdated version in usage 0.1 -> 0.11
• clarified behaviour in COO <-> matrix conversions

why this was needed

The documentation stated:

Upon conversion to other formats, the duplicate entries may be summed together. See the
documentation for the respective conversion functions.

The word "may" is misleading because all conversions from CooMatrix have the behaviour of summing duplicate entries. Since in the future this may not be the case, I kept this wording intact but explicitly highlighted the summing behaviour in the respective conversion methods.