reformat S_scan_heredoc()

reformat S_scan_heredoc()

The indentation was inconsistent and confusing.

Reindent, add blank lines where appropriate, and change this code
comment: "(Closing '}' here to balance"  to '<<}', since vim is far too
clever for its own good these days in terms of using '%' to bounce
between brace pairs.

Should be no functional changes.