docs: improve and compare Promise.each and Promise.mapSeries

[papb]

From the docs for Promise.each:

This article is partially or completely unfinished. You are welcome to create pull requests to help completing this article.

This pull request is a rewrite of the documentation of Promise.each, making it complete (I believe).

Also, Promise.mapSeries is very similar to Promise.each (the former was born in #821 based on Promise.each), so I thought that the documentations of both functions should be more closely related. This way, this pull request also rewrites the documentation of Promise.mapSeries, yielding two similar documentations that clarify the difference between both functions.

Not only rewriting the text, I also changed the examples to ones that I believe to be clearer and more insightful (also better formatted), showing better the differences between both functions.

This is my first contribution, hopefully I've done it right and will be able to help :) Let me know if I missed something or have to further improve something.

[papb]

@benjamingr Sorry to ping for attention, but this PR is probably super easy to review, it is frustrating to take so long. I'm sure you're super busy but can you spare 5 minutes? I would be more than happy to help with other parts of the docs which also say that You are welcome to create pull requests to help completing this article but the delay is frustrating. Thank you so much.

[benjamingr]

@petkaantonov can we do a docs rebuild after this?

[papb]

@benjamingr - Thank you so much for the review! Do we only have to wait now? Is there something else you can do, or we just have to wait more for @petkaantonov ? Thanks again.

[papb]

@benjamingr - Looks like the docs are still old even though this PR was merged one month ago. Can you take a look into this please? Thanks 😬

[benjamingr]

@petkaantonov can you run a docs rebuild (with the release you did)?

[papb]

Ping @petkaantonov

[petkaantonov]

rebuilt docs and pushed