Address PR igerber#344 CI review R3 P3: clarify rank_control_units synthetic_weight docstring

Docstring at prep.py:841 now states synthetic_weight is an informational
heuristic from a single-pass uncentered Frank-Wolfe solve, does NOT factor
into quality_score ranking, and is NOT the canonical SDID unit weight.
Directs users who want canonical SDID weights to SyntheticDiD.fit().

Aligns the public contract with the implementation notes already added in
the prior round.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/prep.py

@@ -838,7 +838,10 @@ def rank_control_units(
         - quality_score: Combined quality score (0-1, higher is better)
         - outcome_trend_score: Pre-treatment outcome trend similarity
         - covariate_score: Covariate match score (NaN if no covariates)
-        - synthetic_weight: Weight from synthetic control optimization
+        - synthetic_weight: Informational heuristic weight from a single-pass
+          uncentered Frank-Wolfe solve; does NOT factor into ``quality_score``
+          (ranking) and is NOT the canonical SDID unit weight. For canonical
+          SDID weights use ``SyntheticDiD.fit()``.
         - pre_trend_rmse: RMSE of pre-treatment outcome vs treated mean
         - is_required: Whether unit was in require_units