x/tools/go/analysis: Pass.All{Package,Object}Facts: document what "all" means

[adonovan]

package analysis // golang.org/x/tools/go/analysis

type Pass struct {
	...
	// AllObjectFacts returns a new slice containing all object
	// facts of the analysis's FactTypes in unspecified order.
	AllObjectFacts func() []ObjectFact

What does "all" mean exactly? I think the intent is that it means the set of facts that you would get if you were to call ImportObjectFact on every types.Object that you are able to obtain from the Pass (and mutatis mutandi for s/Object/Package/g).

More generally the API is unclear as to whether Export(o, f) followed by Import(o) returns f even within the same package. I think the implementations have always done so.