Descriptions for some of bls12* methods

[DariuszDepta]

It would be nice to have some descriptions (in source code) to these methods, what they do and why they are in Api trait:

• bls12_381_pairing_equality
• bls12_381_hash_to_g1
• bls12_381_hash_to_g2

[aumetra]

The pairing equality method already has pretty good documentation IMO. And for the hash function I could only come up with one or two sentences. I'll PR it, maybe you can then have a look at it and let me know where there's gaps so I can fill them out.

I feel like there's the classic problem of "I wrote it and read the papers, so of course it's obvious to me".