doc/OPTIONS: better --cost= examples

Use --cost=0,-3 and --cost=0,10:20 as examples instead
of --cost=:,-3 or --cost=:,10:20

Both versions do the same, but using 0 (meaning: include all costs >= 0)
is probably easier to grasp.
Minor adjustments to john's usage and --list=hidden-options output
doc/README.librexgen: use --regex instead of --rexgen as option name
Merge pull request #574 from frank-dittrich/bleeding-jumbo

Minor adjustments to some doc files and to usage and --list=hidden-options output