Clarification around returning -0.0 from certain builtins

I was wondering if the specification could clarify whenever it says `returns 0.0` whether this means strictly `+0.0` or whether `-0.0` is possible?

This appears to apply to the `sign`, `step` and `smoothstep` builtins but admittedly I only really have `sign` in mind. A more efficient implementation of `sign` may be possible in some cases if the sign was able to be copied directly from `x`, in which case negative NaNs would produce `-0.0`.

My interpretation of `sign` would be that it returns `+0.0` but since the documentation goes out of its way to specify a return value of `+0.0` and `-0.0` when `x` is either of those values, leaving in `0.0` without an explicit sign as an option could be misinterpreted.

The OpenCL-CTS currently passes for an implementation of `sign` that returns `-0.0` for `-NaN`. I don't know if those specific input values are being tested, but either way, if `-0.0` is not intended then it is currently being allowed by omission.

Review the OpenCL specification wording for the sign, step, and smoothstep builtins, then compare it with the OpenCL-CTS behavior described in the issue. The clarification is complete when the specification explicitly states whether an unspecified `0.0` may be `-0.0`, consistently covering the cited inputs and builtins.

Written by the indexing model from the issue text.