Additional documentation/examples related to transaction deadlock

[sparr]

https://github.com/etcd-io/bbolt#transactions provides some warnings about potential deadlock scenarios. However, this documentation uses a lot of uncertain language like "should", "generally", "can", etc.

It would be useful to have more certainty around these scenarios. More explicit narrative documentation could help, but my preference would be concrete examples.

e.g. at least "These example snippets will always cause a deadlock" and "... will never cause a deadlock", possibly also "... will cause deadlock if X additional condition is [not] met", with enough examples of each to cover a variety of cases of multiple transactions, threads, etc.