Unit tests now check that docs/*.txt help examples are all up-to-date.
I ran into a problem here in that the terminal_width needed to be more
accurately defined - so I replaced update-docs-help.sh with update-docs-
help.py which hard-codes the terminal width.
Also introduced a new mechanism for ensuring the --help examples in the
documentation reflect the current output of the --help commands, via a new
update-docs-help.sh script. Closes#336