help: document sharing of revlog header with revision 0

The previous docs were incorrect about there being a discrete header
on revlogs.

mercurial/help/internals/revlogs.txt

@@ -31,7 +31,8 b' File Format'
 -----------
 
 A revlog begins with a 32-bit big endian integer holding version info
-and feature flags.
+and feature flags. This integer is shared with the first revision
+entry.
 
 This integer is logically divided into 2 16-bit shorts. The least
 significant half of the integer is the format/version short. The other
@@ -70,8 +71,10 b' 00 02 00 01'
 00 03 00 01
    RevlogNG + inline + generaldelta
 
-Following the 32-bit header is *index* data. Inlined revision data is possibly
-located between index entries. More on this layout is described below.
+Following the 32-bit header is the remainder of the first index entry.
+Following that are remaining *index* data. Inlined revision data is
+possibly located between index entries. More on this layout is described
+below.
 
 RevlogNG Format
 ---------------
@@ -83,6 +86,8 b' or between index entries (as opposed to '
 Each index entry is 64 bytes. The byte layout of each entry is as
 follows, with byte 0 being the first byte (all data stored as big endian):
 
+0-3 (4 bytes) (rev 0 only)
+   Revlog header
 0-5 (6 bytes)
    Absolute offset of revision data from beginning of revlog.
 6-7 (2 bytes)
@@ -120,6 +125,9 b' If revision data is not inline, then raw'
 separate byte container. The offsets from bytes 0-5 and the compressed
 length from bytes 8-11 define how to access this data.
 
+The first 4 bytes of the revlog are shared between the revlog header
+and the 6 byte absolute offset field from the first revlog entry.
+
 Delta Chains
 ------------
 
@@ -190,4 +198,4 b' hash of a revision:'
 1. Hash the parent nodes
 2. Hash the fulltext of the revision
 
-The 20 byte node ids of the parents are fed into the hasher in ascending order.
  No newline at end of file
+The 20 byte node ids of the parents are fed into the hasher in ascending order.