Update Service Account descriptions

[ltvolks]

• Updates service account descriptions w/ current roles required
• Fixes build warning:

- docs-gcp/docs/service-blocks/index.rst:19: WARNING: Title underline too short.

Platform Essentials
^^^^^^^^^^^^^^^^

[catlook]

some suggested edits

[ltvolks]

Inline suggestions updated

[catlook]

👍

[upodroid]

LGTM. I read the entire docs btw there are several things in there that need updating:

• https://developer.rackspace.com/docs/fanatical-support-gcp/support/#phone
  The numbers are out of date and the link is broken.
• https://developer.rackspace.com/docs/fanatical-support-gcp/security/#security-updates
  This security advisory is from 3 years ago and has been patched a long time ago.
• https://developer.rackspace.com/docs/fanatical-support-gcp/billing/
  Billing file exports are deprecated and shouldn't be recommended anymore
• https://developer.rackspace.com/docs/fanatical-support-gcp/billing/#native-gcp-project-billing-account-access
  Mention the Google role Billing Account Administrator instead of billing:admin
• https://developer.rackspace.com/docs/fanatical-support-gcp/gcp-projects/offboarding/
  Do we want to make the customer steps of the unboarding public?
• https://developer.rackspace.com/docs/fanatical-support-gcp/support/#runway-service-level
  This needs to be removed or clarified.