Add documentation to repo retrieve
Add documentation to repo retrieve

This actioin was added when repos got identifiers. It was not
documented, though.

PDC-1071

tzhaoredhat/automation,xychu/product-definition-center,product-definition-center/product-definition-center,xychu/product-definition-center,release-engineering/product-definition-center,release-engineering/product-definition-center,lao605/product-definition-center,lao605/product-definition-center,tzhaoredhat/automation,tzhaoredhat/automation,release-engineering/product-definition-center,pombredanne/product-definition-center,lao605/product-definition-center,pombredanne/product-definition-center,product-definition-center/product-definition-center,lao605/product-definition-center,product-definition-center/product-definition-center,product-definition-center/product-definition-center,pombredanne/product-definition-center,tzhaoredhat/automation,release-engineering/product-definition-center,pombredanne/product-definition-center,xychu/product-definition-center,xychu/product-definition-center