docs: stop labelling current behaviour as legacy

Three of the five comments the audit found. always() accepts a predicate or a
formula and both are live: the shipped default properties pass a predicate, the
six example-spec properties pass a formula. String selectors are documented
first-class beside object selectors in spec-language.md. "all" is the
--platform flag's default value, not an older calling convention.

cmd/sanderling/doctor.go

@@ -24,8 +24,9 @@ type doctorCheck struct {
 }
 
 // doctorChecksFor returns the host-readiness checks for a target platform.
-// "all" returns the union (deduped by name) so the legacy zero-arg `doctor`
-// behaviour keeps surfacing every platform's prerequisites.
+// "all" returns the union (deduped by name), which is what `doctor` with no
+// platform does: it is the default, so it has to surface every platform's
+// prerequisites.
 func doctorChecksFor(platform string) []doctorCheck {
 	switch platform {
 	case "web":