Get "stopped" back into repr(Thread) when appropriate.

Due to recent changes, a Thread doesn't know that it's over before
someone calls .join() or .is_alive().  That meant repr(Thread)
continued to include "started" (and not "stopped") before one of
those methods was called, even if hours passed since the thread
ended.

Repaired that.
Document Fraction's numerator and denominator properties.
Addresses issue #18800
merge from 3.3

Document Fraction's numerator and denominator properties.
Addresses issue #18800
Clarify mmap.close method  behavior.  Addresses issue  #18815
Patch contributed by Anoop Thomas Mathew.
merge from 3.3

Clarify mmap.close method  behavior.  Addresses issue  #18815
Patch contributed by Anoop Thomas Mathew.