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: 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`