document use of collections with cbworkloadgen

Referring to this doc:

https://docs.couchbase.com/server/7.0/cli/cbtools/cbworkloadgen.html

It does not describe how to generate docs on collections.  Looking at source code it is possible with -c.  However you have to specify collection id in hex.  For example:

cbworkloadgen -n localhost:8091 -b travel-sample -c 0x18 -u admin -p password -i 1000 -j -r 1

You can get the collection id with this command:

/opt/couchbase/bin/cbstats -u admin -p password -b travel-sample localhost:11210 collections | grep ":name:"