7961: add user docs for ssr assist r=JoshMcguigan a=JoshMcguigan
@matklad
This is a small follow up on #7874, adding user docs for the SSR assist functionality. Since most other assists aren't handled this way I wasn't sure exactly how we wanted to document this, so feel free to suggest alternatives.
Co-authored-by: Josh Mcguigan <joshmcg88@gmail.com>