Include do.call(merge, eapply(env, FUN)) example in getSymbols documentation

`?getSymbols` should include an example of the "canonical" way to work with multiple symbols returned by `getSymbols`. This will become more important in version 0.5.0, when the `auto.assign` default will change from `TRUE` to `FALSE`.

A good starting point would be [Jeff's answer on StackOverflow](http://stackoverflow.com/a/5574836/271616), but it would be nice to have one or two examples that are a bit more complicated.