FIP-0095: Document existing get_beacon_randomness syscall behaviour

[anorth]

Adds non-normative description of the existing beacon lookup behaviour.

Thanks to @rvagg for doing the grunt work here to describe it all in https://github.com/filecoin-project/FIPs/discussions/1051#discussioncomment-10494145 and https://github.com/filecoin-project/lotus/issues/12414#issuecomment-2320034935.

• [x] Land #1048 first and rebase

[jennijuju]

Thanks for adding