doc: clarify that optional attrs in a function argument will be ignored unless specified

In `args@{ a ? 1 }: /* ... */` the value `a` won't be a part of `args`
unless it's specified when calling the function, the default value will
be ignored in this case.

My personal point of view is that this behavior is a matter of taste, at
least I was pretty sure that unmatched arguments will be a part of
`args@` while debugging some Nix code last week.

I decided to add a warning to the docs which hopefully reduces the
confusion of further Nix developers who thought the same about `args@`.

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