Issue #17401: document closefd in io.FileIO docs and add to repr

Issue #17401: document closefd in io.FileIO docs and add to repr

closefd was documented in the open docs but not the matching FileIO
class documented. Further, closefd, part of the core state for the
object was not shown.

In review it was noted that the open docs are a little confusing about
the interaction between closefd and paths, so tweaked them at the same
time.