DOC: integrate: move the quad_explain() text to quad() docstring

There's no real reason to keep the documentation in a different place.
DOC: optimize: reformat show_options + fix links to it

Make show_options look better in HTML, and make the links to it
clickable.
Merge pull request #2970 from pv/quad-docc

DOC: fix up quad() and show_options() docstrings