doc/ref: the subsumption relation for fields was backwards

Intuitively, a regular field is more concrete than a required field,
as unifying `{a: x}` with `{a!: x}` results in a regular field,
like unifying `3` with `int` results in `3`, as it is more concrete.

Similarly, a required field is more concrete than an optional field.
This is because unifying `{a!: x}` with `{a?: x}` results in
a required field, which makes intuitive sense - the empty struct `{}`
can unify with `{a?: x}` but not with `{a!: x}`.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I8897ed4356514bb32990797fe3e5bbab783303ff
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194644
Reviewed-by: Paul Jolly <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

doc/ref/spec.md

@@ -1096,7 +1096,7 @@ as long as it is not unified with a defined field.
 
 The subsumption relation for fields with the various markers is defined as
 ```
-{a?: x} ⊑ {a!: x} ⊑ {a: x}
+{a: x} ⊑ {a!: x} ⊑ {a?: x}
 ```
 for any given `x`.