collections.Counter doc does not list added version: http://docs.python.org/library/collections.html It appears to only have been added in 2.7 (while the rest of the doc says it is valid since 2.4)
The convention is to put the note at the end of the section. For this specific case the section about Counter is immediately followed by the documentation of its methods, so it's indeed hard to notice. Maybe a different CSS would make this more evident.
It doesn't seem like the styling is the issue, but the placement. You say that the standard style is to put this at the end of the section, is there somewhere it would be appropriate to bring this up for discussion? I think it would be much more intuitive if it was always placed right after the section name.
FWIW, the version added is also prominently shown at the top of the page: http://docs.python.org/library/collections.html#module-collections I don't think the CSS should be changed or that versionadded should become more prominent -- this is usually one of the least important parts of the documentation for a function or class.