RwTxn + RoTxn documentation improvement.

[darnuria]

Hello!

Following #150, I feel like it was a good idea to document the possibles errors raisable by: http://www.lmdb.tech/doc/group__mdb.html#gad7ea55da06b77513609efebd44b26920

I didn't included ENOMEM since there is no variant yet maybe it's an other whole story.

[darnuria]

Once the other documentation PR lands. Could you please rebase this one on main?

Sure will squash + add co-author-by since you corrected a lot of typo !

[darnuria]

Oh forgot the osx semaphore limitation doing a last minute push.

src: https://github.com/meilisearch/heed/issues/135#issuecomment-1313630140

Done

[Kerollmops]

Thank you very much for the documentation ❤️