Document problems with multi-threading

[mrbean-bremen]

• also fix some minor documentation issues
• see Question: Is ThreadPoolExecutor a known incompatible library? #1102

Tasks

• For documentation changes: The Read the Docs preview builds and looks as expected

[mrbean-bremen]

@schuylermartin45 - please check if the documentation changes are sufficient.

[schuylermartin45]

@mrbean-bremen I read it and I think this is a pretty good improvement that should hopefully make things clear for other users. Thanks!

[schuylermartin45]

I think between these changes and our other discussions, we've left enough bread crumbs on the internet for the next person to run into this.