revlog: document the `is_inline_index` method

A small docstring does not hurt.

mercurial/revlog.py

@@ -291,6 +291,7 b' class revlog:'
 
     @staticmethod
     def is_inline_index(header_bytes):
+        """Determine if a revlog is inline from the initial bytes of the index"""
         header = INDEX_HEADER.unpack(header_bytes)[0]
 
         _format_flags = header & ~0xFFFF