Document R7 P1 + R8 P1 fit-time guards in SyntheticDiD.fit() docstring

The ``Raises`` section of ``SyntheticDiD.fit()`` previously only listed
"non-pweight survey design" among its ``ValueError`` conditions. The
two fit-time guards added in PR #355 R7 and R8 are not reflected in
the docstring:

- R7 P1: zero total survey mass on either arm.
- R8 P1: ``fpc`` declared without explicit ``psu=`` where ``fpc``
  is below the (per-stratum) unit count.

Both raise ``ValueError`` before the bootstrap loop is dispatched.
Expanding the ``Raises`` section makes the contract explicit to
readers (and AI reviewers) cross-referencing the documented behavior
against the implementation.

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

Author: igerber

diff_diff/synthetic_did.py

@@ -270,7 +270,21 @@ def fit(  # type: ignore[override]
         ------
         ValueError
             If required parameters are missing, data validation fails,
-            or a non-pweight survey design is provided.
+            or a non-pweight survey design is provided. Under survey
+            designs, also raises when:
+
+            - The total survey mass on either arm is zero
+              (``w_control.sum() == 0`` or ``w_treated.sum() == 0``).
+              Every unit on that arm would have weight 0, encoding an
+              unidentified target population (PR #355 R7 P1).
+            - ``survey_design`` declares ``fpc`` with no explicit
+              ``psu=``. SDID Rao-Wu then treats each unit as its own
+              PSU, so ``fpc`` must be ``>=`` the number of units
+              (unstratified) or ``>=`` the per-stratum unit count
+              (stratified). Front-door checked after
+              ``collapse_survey_to_unit_level`` so the user sees a
+              targeted error instead of a bootstrap-exhaustion
+              failure (PR #355 R8 P1).
         NotImplementedError
             If ``survey_design`` with strata/PSU/FPC is provided with
             ``variance_method='placebo'`` or ``'jackknife'``. Bootstrap