Clarify usage of gitlab-rails runner

Problem to solve

rails runner example in the support cheat sheet could use clarification around executing scripts and why gitlab-rails requires permissions to access the file, i.e. permission drop to $gitlab_user

ETA: The section on rails runner has since been moved over to a section on the debug page.

Proposal

add an example

gitlab-rails runner /tmp/my-script.rb

clarify that gitlab-rails runner drops permissions to git user, and should have access to /path/to/script.rb as that user.