consider adjusting search --threshold default value and how -n works

[ctb]

see #2168 - a few ideas:

• remove -n completely, 'cause it's a bit silly;
• increase -n to be something larger than three;
• adjust docstring for -n to point at --threshold
• have -n lower threshold automatically;

[ctb]

also ref #2002

[ctb]

see also #1914

[ctb]

Addressed this a bit, and fixed #1914, in #2222. I think #2002 is a good long-term future for search, tho.