Refactor warning message and link on macOS binary wheel merge (#2397)

Author: alexaatm

changes/1650.misc.rst

@@ -0,0 +1 @@
+Improve warning message for unmergeable files in non-universal wheels to clarify fallback behavior and link to documentation.

src/briefcase/platforms/macOS/utils.py

@@ -243,7 +243,9 @@ def merge_app_packages(
                             ):
                                 self.console.warning(
                                     f"{relative_path} has different content "
-                                    f"between sources; ignoring {source_app_packages.suffix[1:]} version."
+                                    f"between sources; ignoring {source_app_packages.suffix[1:]} version. "
+                                    f"This is usually safe if the file content is not used at runtime. "
+                                    f"See https://briefcase.readthedocs.io/en/stable/reference/platforms/macOS/index.html#inconsistent-content-in-non-universal-wheels for more details."
                                 )
                         else:
                             # The file doesn't exist yet; copy it as is (including