pure-parsers: document index class constants

This also adds the big endian prefix `>` to make the constants truly
platform-independent, even if no issue with this has been reported in the wild.

Differential Revision: https://phab.mercurial-scm.org/D10104

mercurial/pure/parsers.py

@@ -42,10 +42,15 b' def offset_type(offset, type):'
 
 
 class BaseIndexObject(object):
+    # Format of an index entry according to Python's `struct` language
     index_format = b">Qiiiiii20s12x"
-    big_int_size = struct.calcsize(b'Q')
-    int_size = struct.calcsize(b'i')
+    # Size of a C unsigned long long int, platform independent
+    big_int_size = struct.calcsize(b'>Q')
+    # Size of a C long int, platform independent
+    int_size = struct.calcsize(b'>i')
+    # Size of the entire index format
     index_size = struct.calcsize(index_format)
+    # An empty index entry, used as a default value to be overridden, or nullrev
     null_item = (0, 0, 0, -1, -1, -1, -1, nullid)
 
     @property