doc/ref: alternative alternative spec for default values
doc/ref: alternative alternative spec for default values

See jba’s https://cue-review.googlesource.com/c/cue/+/1341.

Change-Id: I405624fa2d41c8dbeb4fb5f2788f3da84c51623d
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1660
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@google.com>

cuelang/cue