Document that PTHREAD_POOL_SIZE accepts JS expressions

[RReverser]

PTHREAD_POOL_SIZE was made to accept arbitrary JS expressions back in #10263, but not documented.

I feel it's important to call out in docs that you can use navigator.hardwareConcurrency in that context as that's, arguably, one of the most useful values for multi-threaded apps.

[kleisauke]

Note that navigator.hardwareConcurrency is not available on Node. So perhaps it should be documented that one can use _emscripten_num_logical_cores()?

emscripten/src/library_pthread.js

Lines 561 to 566 in 5dac884

	emscripten_num_logical_cores: function() {
	#if ENVIRONMENT_MAY_BE_NODE
	if (ENVIRONMENT_IS_NODE) return require('os').cpus().length;
	#endif
	return navigator['hardwareConcurrency'];
	},

[RReverser]

Oh, good idea. Is it available from a global scope though / can you use it in that param?

[kleisauke]

It's available in the global scope, you only have to precede that function with _. I'm using it without problems here. However, I'm not sure how it behaves with the closure compiler (haven't tested that yet).

[RReverser]

Hmm I guess it would work but @kripken might know more.

[kripken]

Yes, that should be safe to do, good idea! You'd also need to export it though so it's included in the build, I think.

[RReverser]

Showed this feature to @bmeurer today and sounds like he would appreciate documentation too :) If nobody gets around, I'll document this myself after [undefined time spent on existing tasks].