Add section explaining 'yield' in list comp/generator exp
These examples illustrate what for me is one of the most confusing behaviours in the whole of Python:
>>> [(yield x) for x in ('a','b')]
<generator object <listcomp> at 0x7f69d5379ba0>
>>> list([(yield x) for x in ('a','b')])
['a', 'b']
>>> list((yield x) for x in ('a','b'))
['a', None, 'b', None]
A relevant SO question and bug report