When a ConnectionPool is instructed to drop a particular SpecificPool, it supplies a generic PooledConnectionsDropped error as the reason, with no additional context as to why the pool is being dropped (e.g. a topology update discovered by the RSM). We should provide a more helpful reason that indicates what initiated clearing the pool in the first place.

This would be particularly useful when dropping connections from the RSM: https://github.com/10gen/mongo/blob/b9ee73b8a7b9c319fc2891a9429e02bc1849706a/src/mongo/client/streamable_replica_set_monitor.cpp#L508-L509

https://github.com/10gen/mongo/blob/b9ee73b8a7b9c319fc2891a9429e02bc1849706a/src/mongo/executor/connection_pool.cpp#L663