text: Title: [Points] Clarify `Select All` Points layer keybinds
  Body: Regarding Points selection keybindings:
  _Originally posted by @mstabrin in https://github.com/napari/napari/issues/4809#issuecomment-1186945981_
  
  > Currently it is implemented like:
  > `control+a` -> Select all `shown` points in the slice
  > `a` -> Select all `shown` points in the slice
  > `shift+a` -> Select all points in the layer
  > 
  > From a practical point of view it might be beneficial to have 3 options maybe? Like:
  > 
  > `control+a` -> Select all points in the slice
  > `a` -> Select all `shown` points in the slice
  > `shift+a` -> Select all points in the layer
  > 
  > And adjust the print messages accordingly?
  
  The current keybinding information in the Preferences for `A` and the message when pressing it does not specify the `shown` behavior. So at a minimum, we should:
  - Ensure that the `A` keybinding info indicates it selects *shown* points, as does the message.
  
  Otherwise implementing the other modes is optional? `shown` isn't well documented nor accessible in the GUI.
  See also: https://github.com/napari/napari/pull/4833