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.
Merge pull request #3565 from charris/documentation-fixes

Documentation fixes for `basics.io.genfromtxt.rst` and `creation.py`