Update comment for Sphinx Path exclusion

Point to the correct Sphinx bug to explain why we're excluding
`Path` from Sphinx nitpicks.

docs/documenteer.toml

@@ -74,9 +74,7 @@ nitpick_ignore = [
     # the combination of Sphinx extensions we're using confuse themselves and
     # there doesn't seem to be any way to fix this.
     ["py:class", "asyncio.locks.Lock"],
-    # Pydantic FilePath types create a reference to Path that is unresolved
-    # unless Path is imported locally, but the import is removed because the
-    # symbol isn't referenced.
+    # See https://github.com/sphinx-doc/sphinx/issues/13178
     ["py:class", "pathlib._local.Path"],
     # Cannot be represented by intersphinx.
     ["py:obj", "safir.pydantic._validators.validate_exactly_one_of.<locals>.validator"],