doc/ref/spec.md: write out spec for aliases and optional field sets
doc/ref/spec.md: write out spec for aliases and optional field sets

This is really to complete the previous CL.
I'll probably merge them before submitting.

Change-Id: If818d60b823bd0a61aa7569b8e19a2daafdbb350
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3821
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue