docs: fix stale error-trace output format + markers

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.

docs/error-handling.md

@@ -317,11 +317,16 @@ v := parse(s) catch e {
 ```
 
 ```
-error trace:
-  raised error.BadDigit
-    at parse_digit (parse.sx:12:5)
-    at parse_int   (parse.sx:34:13)
-    at handle_line (main.sx:21:8)
+error return trace (most recent call last):
+  parse_digit at parse.sx:12:5
+        c := s[i] or raise error.BadDigit;
+                     ^
+  parse_int at parse.sx:34:13
+        try parse_digit(s);
+        ^
+  handle_line at main.sx:21:8
+        try parse_int(line);
+        ^
 ```
 
 Traces are on by default in debug builds and compiled out in release