Update docs for note on Parsl's ThreadPoolExecutor

[d33bs]

Description

This PR adds notes based on findings from #75 to help guide users on the use of Parsl's ThreadPoolExecutor if/where it may come up. We no longer use this executor as a default so we might consider this to be a bit of an edge case. Generally, the implementation of ThreadPoolExecutor + arrow can vary based on system configuration and memory allocator availability.

Closes #75
References #323

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

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[gwaybio]

Great documentation!

[d33bs]

Thanks @gwaybio !