langref: add example for errdefer |err| sytnax

doc/langref/test_errdefer_capture.zig

@@ -0,0 +1,19 @@
+const std = @import("std");
+
+fn captureError(captured: *?anyerror) !void {
+    errdefer |err| {
+        captured.* = err;
+    }
+    return error.GeneralFailure;
+}
+
+test "errdefer capture" {
+    var captured: ?anyerror = null;
+
+    if (captureError(&captured)) unreachable else |err| {
+        try std.testing.expectEqual(error.GeneralFailure, captured.?);
+        try std.testing.expectEqual(error.GeneralFailure, err);
+    }
+}
+
+// test