Show 'correct' explicit bool value syntax in help

[alimpfard]

Here's my suggestion from #410 to (begin to) resolve it.

[CLAassistant]

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[tomasaschan]

I think I'd prefer if this said [=true|false] or something like that, to clarify that those are the only valid values. Since the implementation special-cases bool values anyway, that should be relatively straightforward to implement, I think. Not sure what the best format in the help is, though - effectively there are four ways to get two values, and that easily gets confusing:

• omit the flag, get the default value
• specify the flag without a value, get true
• specify the flag with a value, either true or false, and get that

Can we somehow convey all those effects in the help output?

[alimpfard]

I called it value as the alternative (with varname != "") would look like [=foo] (also that [=true|false] just seemed too long...but sure, bool is special enough anyway.
Re "specify the flag without a value to get true", not sure...

[tomasaschan]

Thanks for your contribution!