Changes in this release docs

[Jeff Blaine]

Something that has always bothered me about the 'Changes in this
release' docs for new features (generators for instance):  There's
usually no explanation for why the feature was added.  Sure, it's
nice to leave the feature open-ended to not pigeon hole its usage,
but I think (and maybe this is just me) people are likely to make
better use of the language's new features if they "get it" more
directly through an explanation of the hole that was filled in
the language.  That is, someone requested it and had a reason to
want it.  What was that reason, in real-world terms?

I don't think it's reasonable to expect language _users_ to keep
up with PEPs, etc.

Just a humble request for consideration.