Update time complexity of LRANGE

LRANGE for LinkedLists traverses from nearest end and not always HEAD (as with ZipLists). Tries to make this behaviour clear.  

A client actually asked us about LRANGE -10 -1 on a million-entries list; we found this nice optimization upon looking at the [source](https://github.com/antirez/redis/blob/unstable/src/t_list.c#L575).
Merge pull request #331 from pritambaral/patch-1

Update time complexity of LRANGE