User Guide: Better explain how to set up STDP

[mbeyeler]

It is still not clear how to set up STDP. The User Guide chapter barely mentions the different setSTDP flavors, and the only code snippet that is shown (on DA-STDP) uses the outdated CARLsim::setSTDP version. It is never explained what setSTDP does compared to setESTDP and setISTDP. We should include clear code examples to show which commands to use in which situations.