document file: say which versions read a file, and the Opened record

The document file's format names itself in its first line
(continuum-document/2) and each record's kind in its first byte. A
record of a kind the reading version doesn't know makes the file
unusable: the node doesn't start on it unless told to move it aside
(--force badDocumentFile). lifecycle.md says so: a file one version
wrote is read only by versions knowing every kind of record in it. The
format's comment names the Opened record a node appends as it starts on
the file, which undoes the clean stop's mark.

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/lifecycle.md b/docs/lifecycle.md
index 900349d..3843646 100644
--- a/docs/lifecycle.md
+++ b/docs/lifecycle.md
@@ -90,7 +90,9 @@ document file holding a document is an error with no override.
   own writes. It is appended to every `--document-append` (default 5s) and
   rewritten after an epoch change, a restart, or when the appended part outgrew the
   full save (at most every `--document-rewrite`, default 1h); a record cut short
-  by a crash is dropped when reading. A node starting from it is ready at once; sync
+  by a crash is dropped when reading. A file is read only by a version that knows
+  every kind of record in it: one a later version wrote is unusable to an earlier one,
+  which doesn't start on it unless told to move it aside (`--force badDocumentFile`). A node starting from it is ready at once; sync
   merges what it missed, or it rejoins a newer epoch. A node writes as a fresh actor
   each start, unless `--keep-actor` and it stopped cleanly (the file marks a clean
   stop, and the mark is undone as the node starts on it, before it decides its actor):