bug#44690: Path to revive buttons that are now skipped in Gnus

[積丹尼 Dan Jacobson]

In Gnus, <TAB> no longer jumps to these buttons.

1.  ( ) text/plain          (*) text/html

We learn

   TAB runs the command forward-button (found in gnus-article-mode-map),
   which is an interactive compiled Lisp function in ‘button.el’.

   Any button with a non-nil ‘skip’ property is skipped over.

That is great, except the user is at a loss as to how to revive such a
property.

No amount of searching in the manual or clicking the menu on the button
reveals how.

Nor does C-u C-x C-=.

There should be a clear path for him to find out how.