Correct and extend documentation on value_series

[rmminusrslash]

https://docs.zmon.io/en/latest/user/alert-ref/alert_reference_functions.html#value_series

1. Bug: the argument listed in the doc (n=) raises an exception. In reality, this argument is not named.
2. Since one can not trial run the function it would be nice to document the return value. The function returns a list of prior check results with the most recent one being at position 0

[mohabusama]

Thanks!
Correct var name (count=). Should be removed anyway until we generate docs from code.