Document the safety issues with the unnamed database

[Kerollmops]

LMDB has an important restriction on the unnamed database when named ones are opened the names of the named databases are stored as keys in the unnamed one and are immutable.

I faced a big bug that triggered a SIGSEGV when copying all of the entries of one unnamed database into another env, heed tried to write the values associated with the opened named databases multiple times, it triggered SIGSEGV sometimes.

I didn't take the time to reproduce this behavior, but we must, at least, document it!