[issue6801] symmetric_difference_update documentation fix

[Georg Brandl]

Georg Brandl <georg at python.org> added the comment:

> Well, in that case I don't understand how it is different from
> difference_update() ?

It's different from difference_update because that takes multiple
arguments. The operator form shows an equivalent and therefore shows how
to write an equivalent of the method with multiple arguments.

> suggesting that you can pipe several sets with -=,
> exactly like what you can do with ^=.

Of course you can put multiple or'ed sets on the right of ^=, but then
you can put every expression there.

> In other words, this whole section is unclear in 
> what "other" is, and why we have "others" sometimes.
< (eg a set or a several sets piped together for example)

I've fixed two other descriptions in r74603, it should be consistent now.

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