docs(feature-flags): correct Organization / Asset relabeling controls

[blakeaowens]

The v3_relabel feature (Organization / Asset relabeling) moved from
environment-driven to DB-backed and self-serve, but the docs still described the
old behavior. This corrects the three stale places.

Changes

admin/feature_flags/PRO__feature_flags.md

• Removed the Organization / Asset Relabeling row from the "Feature
  availability" table. That table lists exceptions to the self-serve rule, and
  relabeling is no longer one of them.
• Added an Organization / Asset Relabeling subsection next to the toggle
  instructions, carrying the facts the removed row was holding: on by default,
  and which surfaces the toggle actually governs.
• Replaced the "Managed by deployment" example. It used
  DD_ENABLE_V3_ORGANIZATION_ASSET_RELABEL, which is no longer env-managed.
  Now uses DD_V3_FEATURE_LOCATIONS, which genuinely is, with the DD_ prefix
  matching what the Feature Flags page renders.

asset_modelling/PRO_hierarchy/asset_hierarchy.md

• "Label Changes" no longer tells on-premise users to set the env var and
  restart, or Cloud users to email Support. It documents both controls and what
  each covers.
• Two adjacent stale claims fixed: the name change is on by default as of 3.0
  rather than opt-in for existing subscriptions, and the pointer for turning it
  off now goes to the section that explains the controls.

The behavior being documented

The toggle governs the Pro UI only. The Classic UI labels, its URLs, and
generated reports read DD_ENABLE_V3_ORGANIZATION_ASSET_RELABEL at process
start, and no restart makes the toggle reach them. Those branches are in
dojo/asset/labels.py, dojo/organization/labels.py, dojo/asset/urls.py,
dojo/organization/urls.py, dojo/metrics/urls.py,
dojo/system_settings/labels.py, and dojo/reports/ui/urls.py, and the setting
is only ever assigned from the environment in settings.dist.py.

Verified by forcing the env var and the DB flag to disagree in a running
instance: the OSS labels returned Asset / Organization (following the
environment) while only Pro's overlay followed the database.

Since both default to on and the stored value was seeded from the env var, they
agree until someone changes one, so the docs tell readers to keep them in sync
if they use both UIs.

Docs only. No code or behavior changes.

🤖 Generated with Claude Code