Improvements for documentation

[bluefoxr]

To include:

• Better explanation of individual treatment
• Improved function documentation of Treat() and Normalise(). Mention normalise functions are n_*()
• Example of distance to target normalisation

[bluefoxr]

Closing this with last commit https://github.com/bluefoxr/COINr/commit/97a214b04b656575f5b2ad2f8273115351f1092e