The trace docs predated the current formatter. Corrected against the real
output (library/modules/trace.sx to_string + examples/expected/1025-errors-
trace-format.stderr):
- error-handling.md: replace the obsolete trace example ("error trace:" /
"raised error.X" / "at func (file:line)") with the real format —
"error return trace (most recent call last):" + per-frame "func at
file:line:col" + source line + caret.
- debugger.md: drop the stale "(planned)" marker on the trace formatter
(it is implemented); the tag-name table note now cites the failable-main
reporter's "unhandled error reached main: error.X" line, not a
nonexistent "raised error.X" trace line.
