§9.7.2.1: Reference §16.4.12 for clarity

[KalleOlaviNiemitalo]

In C# 7.x, it would be useful to add a note to §9.7.2.1 (Ref safe contexts / General):

Note: Safe context for expressions of ref struct type is defined in §16.4.12. end note

This would help a person reading the definition of ref-safe-context from §9.7.2 understand why the constraints are written as applying only to reference variables and not also to ref struct variables.

§16.4.12.1 (Safe context constraint / General) already has a link to §9.7.2:

There are three different safe-context values, the same as the ref-safe-context values defined for reference variables (§9.7.2): declaration-block, function-member, and caller-context.

Reformulated from #837 (comment)