docs(tinybird): clarify healthScoreV2 includes imputed category values (IN-1249)

Signed-off-by: Joana Maia <jmaia@contractor.linuxfoundation.org>

Author: joanagmaia

services/libs/tinybird/datasources/health_score_v2_repo_copy_ds.datasource

@@ -12,7 +12,9 @@ DESCRIPTION >
     dependency health (checked against the repo's own published packages' dependencies, not an average
     across unrelated packages), supply chain integrity (hardcoded 0 — provenance/2FA data not yet piped).
     - `developmentActivityScoreV2` (0-25) — release cadence, commit activity, issue resolution, PR merge health.
-    - `healthScoreV2` (0-100) is the sum of the three categories above, clamped to 100.
+    - `healthScoreV2` (0-100) — sum of measured category scores plus population-median imputation for
+    any NULL category (`categoryMax × median(score/max)` over measured repos), clamped to 100. A
+    category field above may be NULL while still contributing an imputed value to this total (IN-1249).
     - `lifecycleLabelV2` — per-repo lifecycle state (active/stable/declining/inert/abandoned/archived),
     computed via the spec's decision tree (archived flag > abandoned > inert > declining > stable >
     active, first match wins), or NULL when the repo has zero commits and zero issues/PRs in every window