DOC: sparse: clarify CSC and CSR constructor usage

Same treatment as given to COO in 3f19f49b83223e826d2e7d97c7060773a8522a0b.

[ci skip]
Merge pull request #4175 from larsmans/csc-csr-constructor-docstring

DOC: sparse: clarify CSC and CSR constructor usage
Merge pull request #1 from scipy/master

Test