diff --git a/emacs/notmuch.el b/emacs/notmuch.el
index 7b61e9be..1c43d3ee 100644
--- a/emacs/notmuch.el
+++ b/emacs/notmuch.el
@@ -388,8 +388,9 @@ any tags).
 Pressing \\[notmuch-search-show-thread] on any line displays that thread. The '\\[notmuch-search-add-tag]' and '\\[notmuch-search-remove-tag]'
 keys can be used to add or remove tags from a thread. The '\\[notmuch-search-archive-thread]' key
 is a convenience for archiving a thread (removing the \"inbox\"
-tag). The '\\[notmuch-search-tag-all]' key can be used to add or remove a tag from all
-threads in the current buffer.
+tag). The '\\[notmuch-search-tag-all]' key can be used to add and/or remove tags from all
+messages (as opposed to threads) that match the current query.  Use with caution, as this
+will also tag matching messages that arrived *after* constructing the buffer.
 
 Other useful commands are '\\[notmuch-search-filter]' for filtering the current search
 based on an additional query string, '\\[notmuch-search-filter-by-tag]' for filtering to include