Rename (spawn, fork) -> 'start process'

Avoid confusion by not referring to starting another process as 'spawning'. Note that 'spawn' is a process creation method, which is an alternative to forking.

Say 'create child process' instead of 'fork' where it doesn't harm understanding. Although we currently only use the 'fork' method for creating subprocesses, there are reasons for us to support 'spawn' in the future.

More information on forking and spawning: https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods 20e6cd02

Depends on !1334 (merged)