[docs] [issue19047] Assorted weakref docs improvements

[Nick Coghlan]

Nick Coghlan added the comment:

I was initially thinking to myself "I have a source checkout right
here, I should just fix it, even though I'm at work and want to go
home"... and then I realised the potential scope of the fixes needed,
given the longstanding misbehaviours these docs assume still exist :)

(I also just realised the clone on my work system is ridiculously
stale, so even updating it will likely take a while at this point)

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<http://bugs.python.org/issue19047>
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