Fixes in the npyio documentation

Fixes the "see also" section of savetxt, which
described savez as compressing (closes #587 ). Also
replaces all occurences of .npy and .npz to use double backticks.
Some had, some did not, and some had " symbols.
small typo
DOC STY: make npyio docstrings less than 80 chars
Merge pull request #3753 from jarondl/fix-doc-savetxt

DOC: Fixes in the npyio documentation