New semantics of sigma in eigs(A, sigma)

- `sigma==nothing` specifies forward iteration (ARPACK `iparam(7)=1`), and a numeric real or complex sigma specifies inverse iteration with level shift (`iparam(7)=3` or 4 depending on `complexOP`)
Update documentation of eigs

- Specifies _all_ the valid values of the `which` keyword in a table.
- Updates `sigma` to its new default and semantics.
- Clarify interaction of `sigma` and `which` keywords via the type of linear operator that is constructed and passed to ARPACK.