[docs] [issue22374] Replace contextmanager example and improve explanation

[cowlinator]

cowlinator <cowlinator at gmail.com> added the comment:

I would like to second the improved explanation of contextlib.contextmanager, and additionally point out another problem:  
A very important piece of information is missing from the documentation:  how to return data from the contextmanager-wrapped function.  

I had to go look up the source code, which had a wonderful explanation in the comments: https://gist.github.com/enuomi/1385336#file-contextlib-py-L56 

In particular, note that 

@contextmanager
def some_generator(<arguments>):
  yield <return_data>

can be used to return <return_data> to the caller, via

with some_generator(<arguments>) as <return_data>:
  print(return_data)

This information is wholly and completely missing from the contextlib.contextmanager documentation.

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nosy: +cowlinator
versions: +Python 2.7, Python 3.4, Python 3.7

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