Description of problem:
Make up and fix some documentation problems

Version-Release number of selected component (if applicable):
docker-distribution-2.4.0-1.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
1.Man page:
registry or docker-distribution
# man registry
No manual entry for registry
# man docker-distribution
No manual entry for docker-distribution

2.Version info:
change ``unknown`` to something others
# registry -v
registry github.com/docker/distribution v2.4.0+unknown

3.Help:
``Additional help topics`` below is filled with blank

# registry help serve
`serve` stores and distributes Docker images.

Usage: 
  registry serve <config> [flags]
Flags:
  -h, --help=false: help for serve


Additional help topics: