Improve lock_path help and documentation
Improve lock_path help and documentation

Improve help string to state default value and that it's mandatory.
Document set_defaults function.

DocImpact: Document that the value needs to evaluate to a valid path.
So, either environment variable is set or option is set.

Change-Id: I481d2f8107476ba166a18bc56d3b4c1be5ff726f

openstack/oslo.concurrency,varunarya10/oslo.concurrency,JioCloud/oslo.concurrency