keystroke to automatically set synopsis text?

When working in index card view, there’s a tiny button in the upper right which will set the synopsis to the first few lines of text. Is there a keystroke alternative for this action?

Also, it would be much more convenient if this button would work on all selected cards, rather than the 1st one.

Also, it seems this feature should also be accessible from the Document menu as well as from the tiny little button.

The next update makes it available from the Documents menu and applies to all documents selected in the binder - so you will be able to assign a keyboard shortcut via System Preferences, too.