cue: clarify status of zero Path value

It's useful to know that a zero `cue.Path` value is usable,
so document that fact.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I0d4bced16832cb5aaa32c4a5500928f36d69dfba
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201913
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

cue/path.go

@@ -258,6 +258,7 @@ type selector interface {
 }
 
 // A Path is series of selectors to query a CUE value.
+// The zero value corresponds to an empty path.
 type Path struct {
 	path []Selector
 }