DOC: sparse: LIL is *not* efficient at all

... unless the user takes care to pre-sort the items. Naively
constructing, say, a term-document matrix can easily take
quadratic time using LIL.
DOC: sparse: demonstrate toarray, not todense

Also pep8 fixes.
DOC: sparse: example of incremental CSR construction
Merge pull request #3516 from larsmans/sparse-docs

DOC improve scipy.sparse docstrings