Re-add section about NGINX config and init script updates in 8.4->8.5 update doc

These sections were removed by !2766 but:
- even if the NGINX config wasn't modified, it might
  be in future updates so it's better to always have
  it instead of having to remember to add it depending
  on the changes
- the init script update section must be there since
  it's a safe command line that should be run on every update

This fixes https://github.com/gitlabhq/gitlabhq/issues/10031.

/cc @rspeicher @yorickpeterse 

See merge request !2815