Parse a single raw comment string (e.g. /// line or /** ... */) into a DocComment.
Used by .sig loader for Doxygen-style blocks. is_block is true for /** … */.
Parse one or more raw comment strings (e.g. from multiple preceding trivia tokens).
If the combined text contains a block comment (/** ... */), only that block is used
so that a preceding line comment (e.g. // Comment) does not corrupt the doc block.