Docs: Clarify that accessing field beyond the last one throws lint warning.

[Sebastian Carlos]

Hello,

Section "4.2 Examining Fields" of the manual says:

> If you try to reference a field beyond the
> last one (such as ‘$8’ when the record has only seven fields), you get
> the empty string.

I think it would be nice to say that doing so throws a warning when the
linter is on:

> warning: reference to uninitialized field

I think this is significant because the next section of the docs even has
an example
that would show that warning if running it with the linter on:

> awk '{ print $NR }'

Thanks for your time.