The documentation portion of racket#634.

Note that this commit does /not/ add to the documented list of keys
read by Dr Racket, because this commit does not add the behavior of Dr
Dracket doing that and using the comment styles to drive its
(un)comment commands.

This was discussed earlier but got left out.

This is relevant for a tool that allows "filling" comments (i.e.
adding/removing line breaks to fit a maximum column), as does Emacs,
where this is the value of the "comment-continue" variable. For line
comments this can obviously just be set to the same value as
comment-start, but for region comments it may differ (e.g. C++ might
use " *" and Scheme might use " ").

Use item list to break up run-on sentence.

Clarify combination of padding with other elements.