Use a word processor (or a text editor that allows for fixed tab stops). This will also allow insertion of diagrams and other non-textual stuff (think clickable

Use a word processor (or a text editor that allows for fixed tab stops). This will also allow insertion of diagrams and other non-textual stuff (think clickable buttons, for compilation and testing, for example) into source code. See, for an example implementation, BlackBox Component Pascal - Windows only, but runs moderately well under Wine.