clarify "ids" statement query format

* replace agent with "agent and groups"
* note that for anonymous groups, each member must be included
Merge pull request #257 from bscSCORM/ids

clarify "ids" statement query format
Merge pull request #258 from andyjohnson/master

Clarify Section 4.1.5
Merge pull request #10 from adlnet/master

Sync up!