Clarify transceiver input/output power leaves for standard and ZR use cases.

[ejbrever]

Currently, the leaves for transceiver input power and output power are not clear and we should update the descriptions. Additionally there is a "when" statement that prevents the input power from correctly being utilized.

Specifically, this is the "when" statement where it is constrained to standard optics, but the input-power is still relevant for coherent channels to represent the total power (signal+noise): https://github.com/openconfig/public/blob/9b0a82ee10b6a598d124250f4df51a927c5916a9/release/models/platform/openconfig-platform-transceiver.yang#L299