doc/spec: specify default constraint is not implemented

For cue-lang#1109

Signed-off-by: Cuichen Li <cuichli@cisco.com>
Change-Id: Iaa5863923a80fd14e23bd1606e2acba0ee15b4fa
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/533342
Reviewed-by: Paul Jolly <paul@myitcv.io>

doc/ref/spec.md

@@ -1061,7 +1061,8 @@ Expression                             Result (without optional fields)
 ```
 
 A struct may define constraints that apply to fields that are added when unified
-with another struct using pattern or default constraints.
+with another struct using pattern or default constraints (_Note_: default
+constraints are not yet implemented).
 
 A _pattern constraint_, denoted `[pattern]: value`, defines a pattern, which
 is a value of type string, and a value to unify with fields whose label
@@ -1086,14 +1087,15 @@ whose label does not unify with any of the patterns of the pattern
 constraints defined for `a` _and_ for which there exists no field in `a`
 with that label.
 The token `...` is a shorthand for `..._`.
+_Note_: default constraints are not yet implemented.
 
 
 ```
 a: {
     foo:    string    // foo is a string
     [=~"^i"]: int     // all other fields starting with i are integers
     [=~"^b"]: bool    // all other fields starting with b are booleans
-    ...string         // all other fields must be a string
+    ...string         // all other fields must be a string. Note: default constraints are not yet implemented.
 }
 
 b: a & {