[PATCH 2 of 3] help: fix incorrect delta chain field documentation (issue5488)

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1488226910 28800
#      Mon Feb 27 12:21:50 2017 -0800
# Node ID 33cdab923127f930357032db7eb6c4c9441e73ae
# Parent  ded4aedfaffbabce6c083f660fc5feeeeb287f0c
help: fix incorrect delta chain field documentation (issue5488)

The existing documentation was wrong. I was responsible for the error:
I was reading the output of `hg debugindex` when I wrote the docs and
that command prints -1 because that's what revlog.deltaparent()
returns.

diff --git a/mercurial/help/internals/revlogs.txt b/mercurial/help/internals/revlogs.txt
--- a/mercurial/help/internals/revlogs.txt
+++ b/mercurial/help/internals/revlogs.txt
@@ -109,10 +109,13 @@ 12-15 (4 bytes)
 
 16-19 (4 bytes)
    Base or previous revision this revision's delta was produced against.
-   -1 means this chunk holds full text (as opposed to a delta).
-   For generaldelta repos, this is the previous revision in the delta
-   chain. For non-generaldelta repos, this is the base or first
-   revision in the delta chain.
+
+   If the revision points to itself, this chunk holds full text (as opposed
+   to a delta).
+
+   For generaldelta revlogs, the revision is the previous revision in the
+   delta chain. For non-generaldelta revlogs, the revision is the base or
+   first revision in the delta chain.
 
 20-23 (4 bytes)
    A revision this revision is *linked* to. This allows a revision in