DOC: clarify doc re: unsupported keys in savez.
DOC: clarify doc re: unsupported keys in savez.

The previous wording ("When saving dictionaries") could be interpreted
as refering to calls like `savez("foo.npz", {"a": 1, "b": 2})`
(literally "saving a dict to foo.npz"); reword to avoid this possibility
of confusion.

simongibbons/numpy,charris/numpy,mhvk/numpy,numpy/numpy,rgommers/numpy,jakirkham/numpy,charris/numpy,jakirkham/numpy,simongibbons/numpy,jakirkham/numpy,mhvk/numpy,seberg/numpy,simongibbons/numpy,rgommers/numpy,mattip/numpy,rgommers/numpy,mhvk/numpy,numpy/numpy,endolith/numpy,charris/numpy,mhvk/numpy,pdebuyl/numpy,anntzer/numpy,pdebuyl/numpy,endolith/numpy,jakirkham/numpy,mhvk/numpy,seberg/numpy,mattip/numpy,rgommers/numpy,seberg/numpy,charris/numpy,pdebuyl/numpy,anntzer/numpy,numpy/numpy,numpy/numpy,anntzer/numpy,simongibbons/numpy,anntzer/numpy,endolith/numpy,simongibbons/numpy,mattip/numpy,pdebuyl/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,mattip/numpy