-In all cases, a relative FCC directory will be understood to
-specify a directory within the notmuch mail store, (as set by
-the database.path option in the notmuch configuration file).
+If `notmuch-maildir-use-notmuch-insert' is set (the default) then
+the header should be of the form \"folder +tag1 -tag2\" where
+folder is the folder (relative to the notmuch mailstore) to store
+the message in, and tag1 and tag2 are tag changes to apply to the
+stored message.
+
+If `notmuch-maildir-use-notmuch-insert' is nil then the Fcc
+header should be the directory where the message should be
+saved. A relative directory will be understood to specify a
+directory within the notmuch mail store, (as set by the
+database.path option in the notmuch configuration file).