Clarify that admin database collections cannot be dropped in sharded topology only

In 4.7.0, attempting to drop any collection in the admin database produces (due to https://jira.mongodb.org/browse/SERVER-35804) the following error:

> Mongo::Error::OperationFailure: Cannot drop collection in admin database (20) (on localhost:27017)

This error message claims that collections cannot be dropped in the admin database. However, collections in admin database can very well be dropped in standalone and replica set topologies; the prohibition is only applicable in sharded clusters. The current wording was quite confusing to me when I first encountered it.

I suggest explicitly stating that the prohibition only extends to sharded clusters.

Besides admin database, the config database appears to be similarly affected.