Now Im trying to autocomplete the insertion of a doctype, but again no
shortcut will bring what is shown in the screencast.
Not sure what you've seen here, but the (to me) relatively "guessable" approach worked perfectly: set the language to "html" (control-option-shift-h) if it's not already, type "doctype", and hit the tab button. You can then select the doctype you want using either the mouse or the numeric equivalents shown.
(And I guess its even faster for the authors to record a screencast on
certain issues rather than writing about them)
I would guess that you guess incorrectly! (Based on my attempts at (non-TextMate) screencasts. It's harder than you might imagine!)
Cheers,
Paul