Temp Path Documentation for Drupal and issues with conf references

[lcatlett]

Expected behavior

• Drupal docs on https://docs.pantheon.io/guides/filesystem/tmp are up to date and correct

Actual behavior

• Following Pantheon Drupal docs results in errors

Steps to reproduce the problem

$conf[ is not valid for Drupal 8+ sites

Suggested Resolution

• Update any references to $conf[ - should likely be config but Pantheon should validate based on drupal release notes

Additional information