t/41: Improved documentation of the Observable#bind method

[oleq]

Suggested merge commit message (convention)

Docs: Improved documentation of the Observable#bind method. Closes ckeditor/ckeditor5#4887.

[coveralls]

Coverage remained the same at 100.0% when pulling 10d4cac on t/41 into ffc91df on master.

[Reinmar]

Could you move this to the deep dive section? I started moving long API docs like this out of API docs where there are hard to find and hard to read. You'll need to extend it a bit and link to it from some places in https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/framework/guides/architecture/ui-library.html and from API docs.

[Reinmar]

Let's move it to the deep dive section of the documentation.

[oleq]

Move like "leave what I did here + extend it in the guide" or "discard what I did here and just write in the guide"?

[Reinmar]

Like: cut what you wrote, paste it into the new guide, review if it makes sense outside (it will miss a context, so most likely it needs to be extended with some intro), add cross-links between various places.

[oleq]

Ready for another round of review:

• I created a separate deep dive guide about observables,
• I simplified the bind() API docs, referring to the deep dive guide,
• I linked to the guide in various places (requires t/ckeditor5-utils/41: Cross-linked from the "architecture overview" guide to the "deep dive into observables" guide ckeditor5#1289)

[Reinmar]

This is confusing. Especially the new Command( 'thing' ) used later on. We should base the docs on some realistic code and we don't write a code like this.

[Reinmar]

See comments.

[Reinmar]

I'll merge this guide now because I'm basing some other changes in another repo on them. You can commit some fixes for it straight to master, @oleq.