Removed redundant POST request exclusion info

The default ``ExceptionListener::setTargetPath()`` already excludes POSTs/PUTs, so suggesting that people should implement their own listener to do this is a bit misleading.

However, doing this to prevent XMLHttpRequest URIs from being saved is still valuable.
I like all of this! But, this is actually related to logging *in*, not logging out:

1) I go to a page in my browser and it redirects me to `/login`
2) While I'm typing in my user/pass, another tab I have open is making an AJAX request, which is being redirected to `/login` (and which is causing `setTargetPath` to be called).
3) I login, and now I'm redirected to that AJAX URL

So, if you clarify this (i.e. don't talk about logout), we're good! If I'm missing something, please let me know!

Thanks!