update export.md: clarify that constructor proxies are exported

Currently, the docs say:
> A member is eligible if all of the following holds:
> ...
> it is not a constructor, nor the (synthetic) class part of an object,

However, constructor proxies are exported and they are synthetic

related: #11128

Author: unkarjedy

docs/docs/reference/other-new-features/export.md

@@ -69,6 +69,7 @@ A member is _eligible_ if all of the following holds:
    a base class of the class containing the export clause.
  - it is accessible at the export clause,
  - it is not a constructor, nor the (synthetic) class part of an object,
+ - it is a constructor proxy
  - it is a given instance (declared with `given`) if and only if the export is from a _given selector_.
 
 It is a compile-time error if a simple or renaming selector does not identify any eligible members.