docs: a signed null keeps the prev chain going

Review 3.1: a signed null stays as an envelope, so the value written
after it names it in prev and an envelope from before the deletion
can't be put back. api.md now says so.

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/api.md b/docs/api.md
index cf05046..681eb60 100644
--- a/docs/api.md
+++ b/docs/api.md
@@ -297,7 +297,10 @@ The node writes it as its envelope if the authenticated user signed it (`403` if
 for this swarm and this path, with one of its keys, may write there (`403`), and it replaces what is
 there (`prev`, the SHA-256 of the envelope it replaces; `409` if it names another:
 read again and sign anew), and only where values are signed (`409` elsewhere). It
-answers with the value, as a `PUT` does; a signed `null` deletes, with `204`. Under
+answers with the value, as a `PUT` does; a signed `null` deletes, with `204`. It stays
+as an envelope (reads show nothing there), so the value written next names it in
+`prev`: the chain goes on through deletions, and an envelope from before one can't be
+put back. Under
 the policy, every other write reaching a signed value, at it, within it or above it,
 is `409`, and so is the users API's.