DOC: Remove claims of unbiasedness from doc string for std
xref #12230

@TomAugspurger ?
 +1, For reference the `np.std` docstring also clarifies that a `ddof=1 still won't be an unbiased.
 is it worthwhile putting in a link to the wiki on the doc-string?
 Do you mean our wiki or wikipedia? I'd say no either way probably.

Are you OK with merging even though that py35 build failed? I saw you open that other issue.
 Actually should I just say 'sample standard deviation' in `basics.rst` and `computation.rst`, since the correction can be specified by passing `ddof`?
 Well actually the same can be said of variance, so never mind. 
