update export.md: clarify that constructor proxies and members created by an export are also exported
update export.md: clarify that constructor proxies and members created by an export are also exported

dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty