document futures size explosion pitfall and mitigations

[spookyvision]

As explained e.g. here, futures can easily turn into unpleasant memory hog surprises.
One possible easy way to investigate problematic code is to add a future-size-threshold = 100 (or whichever threshold seems reasonable) clippy lint.

Further information: withoutboats: Futures and Segmented Stacks

[eldruin]

We would welcome a PR explaining this. Please feel free to write it if you have the time.

[spookyvision]

I don't feel qualified enough to do a detailed writeup, so would rather see somebody else spearheading this. If nobody else takes it up, I can try and come up with 1-2 paragraphs…