What exactly is `enable_parallelism`?

[ghost]

Hi @scottwinkler, looking at the docs and just browsing through the code, it's not entirely clear what enable_parallelism does. Can you add some more docs on it?

I'm curious because I'm having an issue where I have a couple of shell script resources that destroy at the same time and sometimes they seem get stuck and cause my terraform run to time out. I'm wondering if it could have something to do with the mutex. Any insight is appreciated!