The reference manual currently says that the target of an augmented
assignment "is only evaluated once". As discussed in the issue, this is
imprecise: for a target like y[k], the subexpressions y and k
are evaluated only once, but the subscription itself may be performed
twice — once to fetch the old value and once to store the result.

This can be observed directly:

>>> def ob():  print('ob evaluated');  return d
>>> def key(): print('key evaluated'); return 0
>>> d = [[]]
>>> ob()[key()] += [1]   # ob and key run once, but d[0] is read and then written
ob evaluated
key evaluated

This PR rewords both sentences to say that the subexpressions of the
target are only evaluated once, and adds the y[k] example, following
the wording suggested by @terryjreedy in the issue.

Fixes gh-65557