[Enhancement] Improve curl_multi explaination (L27)

[alainlou]

The description for curl_multi was a little confusing - I think it'd be good to go over a high-level description of the internal working of curl_multi (or maybe the pattern of an asynchronous event loop in general) to help us better understand what's going on.

I know you mentioned that we use asynchronous when multi-threaded is not an option, but I thought maybe this was because curl_multi used pthreads internally more efficiently or something.

There were a few points in the lecture that I felt suggested curl_multi used pthreads internally:
https://youtu.be/CTL6JjFljZw?t=86 (not to be harsh, but this was not only a bad joke, it was confusing too tbh)
https://youtu.be/CTL6JjFljZw?t=395 ("we dispatch all easy handles at once")

Perhaps it would also be good to talk about when and how our callback is actually called by curl_multi.

Some relevant Piazza discussions:
https://piazza.com/class/kebv6ghs3ahfo?cid=613
https://piazza.com/class/kebv6ghs3ahfo?cid=530