revlog: add some comment in the header sections

We are about to add more content so let us organise the existing content first.

Differential Revision: https://phab.mercurial-scm.org/D10302

mercurial/revlogutils/constants.py

@@ -11,11 +11,15 b' from __future__ import absolute_import'
 
 from ..interfaces import repository
 
-# revlog header flags
+### main revlog header
+
+## revlog version
 REVLOGV0 = 0
 REVLOGV1 = 1
 # Dummy value until file format is finalized.
 REVLOGV2 = 0xDEAD
+
+##  global revlog header flags
 # Shared across v1 and v2.
 FLAG_INLINE_DATA = 1 << 16
 # Only used by v1, implied by v2.
@@ -26,6 +30,8 b' REVLOG_DEFAULT_VERSION = REVLOG_DEFAULT_'
 REVLOGV1_FLAGS = FLAG_INLINE_DATA | FLAG_GENERALDELTA
 REVLOGV2_FLAGS = FLAG_INLINE_DATA
 
+### individual entry
+
 # revlog index flags
 
 # For historical reasons, revlog's internal flags were exposed via the