Fix Explainer.md/Binary.md definition of outer aliases

[lukewagner]

This PR intends to address the first issue in this comment, bringing the spec in line with the implementation (and logic).

(cc @zherczeg and @alexcrichton)

[zherczeg]

Thank you for the doc update!

[zherczeg]

Btw, in some cases the explainer refers to "transitive" dependencies. In this case, not just resource types, but types which transitively refer to resources (e.g. a borrow) should be disallowed as well (if I understand this correctly).

E,.g.

   (type $IT (instance (export "sub-res" (type (sub resource)))))
   (import "any:any/anything@1.0.0" (instance $I (type $IT)))
   (alias export $I "sub-res" (type $SUB-RES))
   (type $BORROW (borrow $SUB-RES))
   (component $C (alias outer 1 $BORROW (type)))
   (export "comp" (component $C))

This is the error message of wasm-tools:
error: cannot alias outer type which transitively refers to resources not defined in the current component (at offset 0x56)