DOC : minor changes to linspace docstring

 - added optional flag to dtype
 - moved conditional on step to the description from the type
ENH: Add support for ATLAS > 3.9.33.

Recent ATLAS combines the previous libraries into two

* libsatlas -- single threaded.
* libtatlas -- threaded.

This fix is a bit of hack in that ATLAS > 3.9.33 is treated as a new,
separate library covered by atlas_3_10_info, but the latter derived
from atlas_info, which treats the cblas, atlas, and atlas_lapack
libraries separately, so the new info has a bit of repetition.

The alternative would be to rewrite atlas_info, but that can wait
on a larger cleanup of the build system.

Closes #3774.