Amends b009aca38a861f74fd5378db19c65db286ad397e.
The PR predated the introduction of unused variable/constant checks,
thus the build checks weren't reporting this failure until later when
merged into master.
This is for consistency with the documentation on sentinel-terminated
{arrays,slices,pointers} which already use `N` for a comptime-inferred
size rather than `X`.
Also adds a behavioral test to assert that a string literal is returned.
This was already the case, but the documentation failed to point out
that the returned value is of type `*const [N:0]u8`, i.e. that of a
string literal.
Also adds a behavioral test to assert that this is the case.
