Resolve placebo_effects deprecation version: 3.6.0 -> 3.5.2

Address CI AI-review P3 (consistency note). The placebo_effects ->
variance_effects rename/deprecation (PR #532) ships in THIS release,
which is 3.5.2, not the guessed next-minor 3.6.0. Update the
`.. deprecated::` Sphinx directive accordingly and drop the now-resolved
"assumed next-minor (current is 3.5.1); confirm/resolve at bump-version
time" placeholder comment. The v4.0.0 removal target (stated in the
warning message) is unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: igerber

diff_diff/results.py

@@ -1209,13 +1209,11 @@ def coef_var(self) -> float:
     def placebo_effects(self) -> Optional[np.ndarray]:
         """Deprecated alias for :attr:`variance_effects` (removed in v4.0.0).
 
-        .. deprecated:: 3.6.0
+        .. deprecated:: 3.5.2
             Renamed to ``variance_effects`` because the array's contents are
             method-specific (placebo effects, bootstrap ATT draws, or
             leave-one-out estimates depending on ``variance_method``).
         """
-        # `3.6.0` is the assumed next-minor (current is 3.5.1); confirm/resolve
-        # at bump-version time. The v4.0.0 removal target is fixed.
         warnings.warn(
             "SyntheticDiDResults.placebo_effects is deprecated; use "
             "variance_effects instead. The array holds placebo effects, "