Disabled menu items disable HELP

[Eli Zaretskii]

There's a problem with our code that disables menu items if some
conditions for invoking the respective commands are not satisfied:
there's no way to get help for these menu items (e.g., with C-h c or
C-h k) as long as the menu item is disabled.

I'm not sure how best to solve this, but it looks to me like a serious
problem.  A user who wants to know what a menu item does should be
able to do that even if the command itself cannot be invoked.

Perhaps we should introduce a special variable that, when non-nil,
allows the disabled menu items to send their events to the input
queue.  HELP commands could then bind this variable.