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>