ht: Allow adding comment to type descriptions

This will be used to add some more details to type descriptions, e.g. on
opcode parameters or result values. The implementation is very similar
to “WithDesc”.

I chose to use “[…]” after finding “/*…*/” hard to read and spot. At
some we'll have to introduce proper formatting (e.g. using HTML).

Example with a comment:
  List of ((Length 2) and (Item 0 is (NonEmptyString [name of changed
  parameter]), item 1 is Anything))

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>