docs: Clarify the default state of an Arg/field

[epage]

Inspired by https://www.reddit.com/r/rust/comments/1i5np88/clap_documentation_is_too_confusing_for_me/m87mutm/

I always use derive, and I mostly use the reference. From manual
experimentation I’ve had to develop the mental model that short
implicitly is what makes an argument non-positional, and if I want an
argument to be positional then it will be by default unless I specify
short. When I search for the word “positional” in the reference, there
are no results. When I search for it in the tutorial, there is one
result that says something I can do with an argument that i’ve made
positional, but there’s no indication for how to make the argument
positional in the first place. The reference documentation on short
doesn’t mention anything about the fact that that’s what determines
whether an argument is positional, even when I click through to the
builder documentation on Arg::short.