Related doc bug:
I expect it should be "only one thread at a time". I doubt it's about pinning some operation to a particular thread:-)
However, maybe it should rather be something like "No other transaction doing mdb_open() may exist at the same time, nor a write transaction (since that also updates the databases), nor may such a transaction exist during mdb_close()" ?
I note mdb_drop() also updates the databases and can likely not coexist with the others, but that's covered by mentioning write txns.
Regarding mdb_drop(), while I'm at it: @brief Delete a database and/or free all its pages. @param[in] del non-zero to delete the DB from the environment, otherwise just free its pages. Free which pages? In the DB, or some malloced ones? I expect it should be "delete a database or empty its contents", and "del non-zero ... otherwise just empty the database."