macro expander: fix `identifier-binding` on fully expanded module

Fix the case of an identifier that is used as a binding in a module
but originated from a different module.
The implementation of `#%top` within a `module` has, for a while,
required that the identifier wrapped by `#%top` not have a local
binding. Change the documentation to match the implementation in that
way. (Since local binding in an identifier's lexical information
contributes to its identity as a top-level binding, that specification
of `#%top` would make sense everywhere, but I've left the top level
alone for backward compatibility.)

Also, change `local-expand` to never introduct `#%top`
wrappers. That's a little more consistent with what `#%top` has
evolved to mean, and it specifically works better with
`local-expand/capture-lifts`.

Closes PR 14635 and PR 14654