Documenting the locks when taking a transaction

[Tamo]

It would be nice to document a little bit about what the locks are when taking a transaction;

• The fact that we can't take two write transactions at the same time
• The fact that we can take multiple read transactions while there is a write transaction

https://docs.rs/heed/0.20.0-alpha.0/heed/struct.Env.html#method.write_txn