DOC: add missing parameters to npyio.genfromtxt, fix #2929

`loose` `missing` and `skiprows` were missing from the docstring of
`npyio.genfromtxt`. The later two are actualy deprecated, but were added
nonetheless.
DOC: two minor typos in genfromtxt
Merge pull request #4428 from jarondl/genfromtxt-doc

DOC: add missing parameters to npyio.genfromtxt, fix #2929