Description of problem:
"Chapter 4. Configuration Hooks" of "Advanced Overcloud Customization guide" contain ambiguous examples of "openstack deploy" command: it leaves an impression that the only environment file that should be provided with the command is the file that was changed/added. And this is not true.
A user should append new environment file to a list of original environment files that were used to deploy environment to update existing environment. If user will issue "openstack overcloud deploy" command with only one environment file he will break his existing environment and will be very frustrated.
It is possible to use example format from upstream documentation  (this is current documentation, so everything except example may be incorrect for RHOSP 10) or KCS article 
Follow multiple examples from documentation, issue "openstack overcloud deploy" with modified or created environment file.
A link to reported document: