Add help text for selecting a block with an override

[richtabor]

A follow-up to #61515, let's add help text to provide additional context about the block override, when selecting the block icon (instead of having no action).

Note that we do not have an official "Learn more" link for overrides yet, but this would be a great place to link to additional information.

[richtabor]

When multiple blocks with overrides are selected, let's use this:

These blocks are editable using "Recipe Title" and "Recipe Description" overrides. 

[annezazu]

Is there a reason we are adding this to the toolbar instead of in block settings? I feel like we generally have stayed away from help text in the toolbar.