Miller 5.6.2: Bug fix for CSV/TSV with many files

Bug fixes:

• #271 fixes a corner-case bug with more than 100 CSV/TSV files with headers of varying lengths.

Documentation:

• The new http://johnkerl.org/miller/doc/whyc-details.html is an elaboration on http://johnkerl.org/miller/doc/whyc.html which answers a question posed by @BurntSushi on Reddit a couple years ago which I did not address in detail at the time.