Add dedicated content on determinism and methods vs. functions to the reference manual

[robin-aws]

What change in documentation do you suggest?

Users are frequently confused about the rules around :|, for example. https://www.microsoft.com/en-us/research/wp-content/uploads/2016/12/krml252.pdf contains all the raw content we need, but it should be condensed and streamlined into a single short section, and probably linked to from several other places in the documentation.