DOC: Fixes for doc/source/user/basics.io.genfromtxt.rst.

Add missing part of usecols negative index explanation and other
minor redaction fixes.
MAINT: Cleanup doc/source/user/basics.io.genfromtxt.rst.

Remove trailing whitespace.
Break long lines.
Fix some indentation.
Use the `::` directive to indicate interactive examples.
Fix doc typo
ENH: inv/solve work with empty inner and others empty outer array

This makes the inverse of a 0x0 array simply be 0x0 again. It
also modifies the no-empty array check in favor of a no-empty
*inner* array, since the gufuncs seem to handle the other case
fine.
MAINT: runtests: make it work on Fedora
Merge pull request #3572 from pv/fix-runtests

MAINT: runtests: make it work on Fedora
ENH: For smaller array, added macro NPY_BEGIN_THREADS_THRESHOLDED in ndarraytypes.h
Avoiding NPY_BEGIN_THREADS for small arrays, can speed-up trivial_three_operand_loop by 5%.
As releases of GIL, then quickly restoring just after small operation doesn't benefit.
Merge pull request #3568 from mspacek/patch-1

Fix doc typo
BUG: Boolean assignment allowed writing to 0-sized array

This was because of the assumption that broadcasting works
if the dimension is not > 1, but correct is != 1.

Adepted from a patch provided by prossahl.

Closes gh-3458.
MAINT: fix typo in npy-format.txt
Merge pull request #3576 from mspacek/patch-3

MAINT: fix typo in npy-format.txt
Merge pull request #3575 from seberg/issue-3458

BUG: Boolean assignment allowed writing to 0-sized array
Merge pull request #3521 from arinkverma/gsoc_performance

ENH: Avoiding NPY_BEGIN_THREADS for small arrays can speed-up trivial_three_operand_loop by 5%
Merge pull request #3565 from charris/documentation-fixes

Documentation fixes for `basics.io.genfromtxt.rst` and `creation.py`
Merge pull request #3513 from seberg/0d-linalg

ENH: inv/solve work with empty inner and others empty outer array