lei_store: keywords => vmd (volatile metadata), prepare for labels

Since keywords and mailboxes (AKA labels) are separate things in
JMAP; and only keywords can map reliably to Maildir and mbox;
we'll keep them separate in our internal data representations,
too.

I initially wanted to call this just "meta" for "metadata", but
that might be confused with our mailing list name.  "metadata"
is already used in Xapian's own API, to add another layer of
confusion.

"tags" was also considered, but probably confusing to notmuch
users since our "labels" are analogous to "tags" in notmuch,
and notmuch doesn't seem to cover "keywords" separately...

So "vmd" it is, since we haven't used this particular
three-letter-abbreviation anywhere before; and "volatile" seems
like a good description of this metadata since everything else
up to this point has been mostly WORM (write-once, read-many).

1 files changed, 11 insertions, 2 deletions

diff --git a/Documentation/public-inbox-glossary.pod b/Documentation/public-inbox-glossary.pod
index 61e1e9f8..10b3f9d6 100644
--- a/Documentation/public-inbox-glossary.pod
+++ b/Documentation/public-inbox-glossary.pod
@@ -69,8 +69,8 @@ L<public-inbox-nntpd(1)> or L<public-inbox-imapd(1)>
 
 Private, per-message keywords or flags as described in RFC 8621
 section 10.4.  These are conveyed in the C<Status:> and
-C<X-Status:> headers for L<mbox(5)>, as IMAP FLAGS (RFC 3501 section 2.3.2),
-or Maildir info flags.
+C<X-Status:> headers for L<mbox(5)>, as system IMAP FLAGS
+(RFC 3501 section 2.3.2), or Maildir info flags.
 
 L<public-inbox-watch(1)> ignores drafts and trashed (deleted)
 messages.  L<lei-import(1)> ignores trashed (deleted) messages,
@@ -83,6 +83,15 @@ the same email into one or more virtual folders for
 ease-of-filtering.  This is NOT tied to public-inbox names, as
 messages stored by lei may not be public.
 
+These are similar in spirit to arbitrary freeform "tags"
+in mail software such as L<notmuch(1)> and non-system IMAP FLAGS.
+
+=item volatile metadata (VMD)
+
+For L<lei(1)> users only, this refers to the combination of
+keywords and labels which are subject to frequent change
+independently of immutable message content.
+
 =item IMAP INTERNALDATE, JMAP receivedAt, rt: search prefix
 
 The first valid timestamp value of Received: headers (top first).