Some field tags are undocumented

(Sorry for closing the last pull request https://github.com/jessevdk/go-flags/pull/60 I could not figure out how to make two clean branches with the commits of the master branch)

The field tags long-description, no-flag, ini-name and no-ini are undocumented.

"_read-ini-name" is another tag I found to be undocumented but it looks like an internal temporary value.