feat: add reciprocal to the specification

[kgryte]

This PR

• resolves RFC: add reciprocal for computing the element-wise reciprocal #790 by adding support for computing the reciprocal to the specification.
• requires that special cases be handled as if implemented as 1.0 / x.
• adopts the same "type promotion" language as other unary element-wise APIs, which is based on the premise that an input array promotes to itself. If this language should be changed, we should do as part of a follow-up PR across all relevant element-wise APIs.
• requires ("should") that the input array have a floating-point data type, as the output array should have a floating-point data type and promotion of mixed kinds is left unspecified.