Update docs/shortcuts.md

Co-Authored-By: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

docs/shortcuts.md

@@ -8,6 +8,6 @@ The modifier is <kbd>Ctrl</kbd> on Windows & Linux and <kbd>⌘</kbd> on Mac.
 - <kbd>Ctrl</kbd>/<kbd>⌘</kbd>+<kbd>k</kbd> - jump to named room
 - <kbd>↑</kbd>/<kbd>↓</kbd> - navigate old messages to edit in composer
 - <kbd>↑</kbd>/<kbd>↓</kbd> - next/prev room when focus in room list
-- <kbd>Alt</kbd>+<kbd>↑</kbd>/<kbd>↓</kbd> - scroll timeline up/down ?
+- <kbd>Alt</kbd>+<kbd>↑</kbd>/<kbd>↓</kbd> - resend previous messages when the composer is in focus
 - <kbd>PageUp</kbd>/<kbd>PageDown</kbd> - scroll timeline up/down
 - <kbd>Ctrl</kbd>/<kbd>⌘</kbd>+<kbd>Home</kbd>/<kbd>End</kbd> - jump to timeline start/end