delforward: allow deletion of "unknown in_htlc_id" and fix autoclean to use it.

Note the caveats: we will delete *all* of them at once!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-delforward.7.md

@@ -20,6 +20,11 @@ has no effect on the running of your node.
 You cannot delete forwards which have status *offered* (i.e. are
 currently active).
 
+Note: for **listforwards** entries without an *in_htlc_id* entry (no
+longer created in v22.11, but can exist from older versions), a value
+of 18446744073709551615 can be used, but then it will delete *all*
+entries without *in_htlc_id* for this *in_channel* and *status*.
+
 RETURN VALUE
 ------------
 

doc/lightningd-config.5.md

@@ -461,6 +461,11 @@ accepted, and ignored.
 
   How old invoices which were not paid (and cannot be) (`expired` in listinvoices `status`) before deletion (default 0, meaning never).
 
+Note: prior to v22.11, forwards for channels which were closed were
+not easily distinguishable.  As a result, autoclean may delete more
+than one of these at once, and then suffer failures when it fails to
+delete the others.
+
 ### Payment control options:
 
 * **disable-mpp** [plugin `pay`]