flag: package level nor CommandLine var docs mention default behavior of -h or -help flags

[aaraney]

Out of the box, the default flag.FlagSet variable, flag.CommandLine, will print usage information and exit 0 if -h or -help is given and a flag with that name was not already registered. This nicety feature is great, however I discovered by surprise! Looking through the flag documentation at 1.22.2, I found mention of the -h and -help flags in ErrHelp's documentation, but nothing at the package level or on CommandLine specifically about this default behavior.

ErrHelp is the error returned if the -help or -h flag is invoked but no such flag is defined.

I am happy to open a PR to improve documentation in this area if it would be well receive :).

[seankhliao]

it's part of FlagSet.Parse https://pkg.go.dev/flag#FlagSet.Parse

[aaraney]

The return value will be ErrHelp if -help or -h were set but not defined.

@seankhliao, the above snippet from the documentation describe entirely different behavior. CommandLine.Parse doesn't return anything and will exit 0 if -h or -help is provided. This is undocumented.