Add comment explaining the bitwise operations in the context of Unix …

…permissions

src/pip/_internal/network/cache.py

@@ -77,6 +77,8 @@ def _write(self, path: str, data: bytes) -> None:
             with adjacent_tmp_file(path) as f:
                 f.write(data)
 
+            # `& 0o666` selects the read/write permissions of the directory.
+            # `| 0o600` sets owner read/write permissions.
             os.chmod(f.name, os.stat(self.directory).st_mode & 0o666 | 0o600)
             replace(f.name, path)