Personally, I find the smarty documentation fairly satisfying.
However, if you are not particularly proficient with PHP, that's not a smarty documentation problem, rather a lack of php knowledge problem :)
Plus, I understand that Smarty documentation is heavily based on Unix because, well, that's what PHP was originally meant to run on! Thus, if you use Windows and have never seen Unix/Linux, that might be a problem as well. My suggestion is that, if you're using Windows, you create a Linux virtual machine, something to go by like Ubuntu, RedHat, etc. That will give you all the tools to run php/smarty in the environment it was supposed to work in.
While I agree it will take you a bit more time, I am confident you will not regret this choice on the long run.
I also think Smarty holds up well for large projects. When I was using Smarty to do newer work in the intranet of an Italian bank with over 200 branches, I didn't feel Smarty was limited in any way.