doc: note JavaScript is not single threaded

[benjamingr]

Honestly I appreciate the effort and don't have time to go over the whole thing but there are plenty of inaccuracies in this document.

This PR fixes the first one I noticed - JavaScript as a language is not single threaded. Nor has it ever been, and multithreaded JavaScript execution dates back at least 10-20 years

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[ovflowd]

If I recall this document was a port of the original guide written many years ago, so not sure who even wrote this initially.

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Unit Test Coverage Report

Lines	Statements	Branches	Functions
	90.67% (593/654)	76.08% (175/230)	94.57% (122/129)

Unit Test Report

Tests	Skipped	Failures	Errors	Time
131	0 :zzz:	0 :x:	0 :fire:	5.388s :stopwatch:

[joyeecheung]

I wonder if there's a better way to describe this. What happens in Node.js currently is:

1. In general, each libuv event loop pairs with one JS execution thread. There are 7 threads in total by default: 1 main thread (which executes JS and runs the event loop), 4 V8 task runner threads, 1 V8 delayed task runner thread, 1 inspector I/O thread; If any async operations are initiated, 4 more threads will be spawn by the libuv thread pool, and there will be 11 threads in total. (Sometimes additional threads & event loops are spawned for e.g. watchdog for Ctrl+C signals, but those are on-demand)
2. When we are talking about a single Node.js instance (instances = main instance or Worker thread instances), each of them has 1 JS thread which runs one event loop. Every Worker spawn adds one more thread into the process (e.g. if the application does some async operation in the main thread, spawns a worker, and then also does some async operation in the worker, there would be 12 threads in total)

To emphasize that one can spawn threads from JS, maybe it's clearer to just mention Workers. But when we are talking about event loops, they do have a 1-1 correspondance with JS threads (each worker gets their own event loop). Or we can just drop "despite the fact that JavaScript is single-threaded" completely since it's not exactly clear what this is talking about in this context.