Docs for SERVER-3181: Add option to listDatabases to only get db names, not size info

Add nameOnly option to listDatabases.

Engineering Ticket Description:

I have about 29377 files (including datafile and nsfile), 13666 databases. The total size is about 1TB. The mongod started with parameters "smallfiles" and "noprealloc".
When I run the command listDatabases, it takes too long on calculating "sizeOnDisk" and determining the empty of database, more than a hour sometimes. But I just need a list of database names. An option to not display size or empty probably makes sense.
I found that mongod will receive the listDatabases command in the following cases:
1. Dump all databases use mongodump
2. A Member of replica set doing initialSync
3. Add a new shard to a cluster.
In these cases, listDatabases may cause mongod overload and can not do the normal queries.