Document new user in user facing docs

• Brief
• Implementation Plan

Brief

Document the existence of the non-root, non-default user in user facing documentation.

Implementation Plan

1. We should update our ruleset documentation, and any analyzer that has its own to ruleset configuration samples to set TargetDir to something that is not a root subdirectory but under /tmp. This avoids a permission error.
2. Create a breaking-change notice in the "deprecations and removals" section of documentation.

/cc @amarpatel