Improving Array module documentation

[fmpanelli]

Adding descriptions and examples for several functions of the Array module.
My aim is to make the awesome fp-ts library a bit more accessible to users and to beginners in particular.

No changes have been done to the actual code.

Thanks in advance for every feedback and input for additional improvement.

Filippo

[grossbart]

Thanks, this is very helpful, I think! Please have a look at my comments and fix them, if you agree. After that I would suggest to merge this.

[fmpanelli]

Included all improvements suggested by @grossbart , thank you!