Write documentation for all the LIBPROCESS_* environment variables.

libprocess uses a set of environment variables to modify its behaviour; however, these variables are not documented anywhere, nor it is defined where the documentation should be.

What would be needed is a decision whether the environment variables should be documented (a new doc file or reusing an existing one), and then add the documentation there.

After searching in the code, these are the variables which need to be documented:

1. LIBPROCESS_IP
2. LIBPROCESS_PORT
3. LIBPROCESS_ADVERTISE_IP
4. LIBPROCESS_ADVERTISE_PORT