docs: describe short md5 handling as the code does

No platform YAML holds an md5 shorter than 32 characters: the Batocera scraper completes one when a single collection md5 starts with it. The pages said some entries carry 29 characters and that scrapers store them as-is.

tests/test_resolution_evidence.py

@@ -86,7 +86,7 @@ class DeclaredHashVerdict(unittest.TestCase):
         )
 
     def test_a_truncated_md5_still_decides(self):
-        """Batocera publishes 29-character prefixes, which are still evidence."""
+        """An MD5 shorter than 32 characters is still evidence, by prefix."""
         self.assertEqual(
             self.verdict(self.right, md5_list=[self.d["md5"][:29]]),
             "data_dir_hash_exact",