Un-break port, fix inaccurate description

Detailed log:

• Unbreak port: Add missing file ${FILESDIR}/mongos.in
• Corrected description to reflect that this port supports RocksDB as a backend AND all other standard MongoDB backends (which for now is MMAPv1 and WiredTiger).
• Bump PORTREVISION

PR: 219259
Submitted by: numisemis@yahoo.com (maintainer)