DOC: sparse.linalg: typo
ENH: sparse.linalg: take LinearOperator in svds

Should allow solving PCA for sparse matrix.
DOC: advertise svds(LinearOperator) + vanity
BUG: sparse.linalg: fix up svds for complex-valued linearoperators

Also fix the tests to sort the singular values, since they are not
necessarily in order.
Merge pull request #4184 from pv/pr-4074

ENH: Support LinearOperator as input to svds