docs: document year=-1/month=-1 sentinel values for lab cross samples

Closes #1092

Add a notes section to the sample_metadata() @doc decorator explaining
that some samples are lab crosses (mosquitoes bred in the laboratory)
that use year=-1 and month=-1 as sentinel values for 'no collection date'.

Include an example showing how to filter them out using sample_query.

This is a documentation-only change — no runtime behavior is modified.

Author: khushthecoder

malariagen_data/anoph/sample_metadata.py

@@ -702,6 +702,17 @@ def clear_extra_metadata(self):
     @doc(
         summary="Access sample metadata for one or more sample sets.",
         returns="A dataframe of sample metadata, one row per sample.",
+        notes="""
+            Some samples in the dataset are lab crosses — mosquitoes bred in
+            the laboratory that have no real collection date. These samples
+            use ``year=-1`` and ``month=-1`` as sentinel values. They may
+            cause unexpected results in date-based analyses (e.g.,
+            ``pd.to_datetime`` will fail on negative year values).
+
+            To exclude lab cross samples, use::
+
+                df = api.sample_metadata(sample_query="year >= 0")
+        """,
     )
     def sample_metadata(
         self,