Modify man pages for image-naming

This PR adds recommendations in man pages to use only [a-z0-9-_.] when
naming and tagging images.  The purpose of this is to add consistency
and to make image naming caps rules seem less arbitrary.

This PR addresses confusion with:
1. BaseImage:Tagged (not allowed)
2. baseimage:Tagged (allowed)
3. baseimage/tagged:V1 (allowed)
4. baseimage/Tagged:V1 (not allowed)

Signed-off-by: Sally O'Malley <somalley@redhat.com>