bug#48504: Improve style of C-h t description (Preface)

[Sebastian Urban]

> > > We didn't yet introduce the Ctrl etc. notation at that place.
> >
> > What about "Control" instead of "Ctrl"?
>
> You mean, just capitalize what the text already says? no objections
> here.

No, I mean to change this:
   (which is ``control h and then t'')
into this:
   (hold down @key{Control} and type @kbd{h}, then let go of
   @key{Control} and type @kbd{t})