cmd/go: do not print entire help text for unrecognized flag
I typed 'go list -josn' without realizing I'd mistyped json, and I was confused for
quite a while as to why I was staring at the 'go help json' text: the actual problem
(a missing flag) scrolls far off the screen. If people want the full text, they can
easily ask for it, but don't drown the important bit - unrecognized flag or other
improper usage - with pages of supporting commentary. The help text does not
help people who just need to be told about a typo.