fetch callback needs documented requirements

[vinniefalco]

The documentation needs to be explicit about the restrictions and limitations of what can be performed in the fetch callback:

From https://www.reddit.com/r/cpp/comments/60px64/nudb_100_released_a_keyvalue_database_for_ssds/dfaj3z7/

db.insert(the_key, ...);
db.fetch(the_key, [&](void const* buffer, std::size_t size) {
    unexpectedly_slow_operation();
    // wow that really took a long time
    //
    // did you know that we're still holding the internal rwlock?
    //
    // it's "never held during disk reads and writes"
    // but our request was fetched from the write queue,
    // which is probed by every reader and writer in order to
    // maintain consistency
    //
    // now guess who is currently waiting to get exclusive access...
    // ...it's the background worker!
    //
    // Q: are other readers still able to acquire the rwlock?
    // A: of course not, we don't want writer starvation :)
    //
    // TL;DR: a reader can block the whole system forever ... :D
    db.fetch(some_other_key, [&](...) { /* let's deadlock */ });                
}, ...);