A case could be made that the "eval" source is bogus, and it was
better to omit it. But the new expander's source reporting is more
consistent, as reflected by the fact that "eval" shows up in a syntax
error with both the old and new expanders.
If a value is convertible to 'text, then use that conversion.
Otherwise, convert using `write` instead of always using "???".
Also, correct documentation to include convertible values among
the valid forms of content, and document the new conversion rules
there.
I'm not certain that mixing 'grouper and non-'grouper parts at a given
level makes sense. Still, this adjustment brings Scribble section
counting more in line with Latex, so that section links are less
likely to have the wrong number label.
