RFC: Expands array doc

The docs are currently very sparse (sic) on the matrix factorization objects, the special matrix types, and the relationship between dense matrix methods and their sparse counterparts.

To make a first stab at addressing these lacunae, this pull request provides:

- Table of available matrix factorizations
- Tables of available special matrix types and availability of optimized methods
- Table of correspondence between dense and sparse matrix methods
- Minor speeling fixes