langref: add example for errdefer |err| sytnax

doc/langref.html.in

@@ -2984,6 +2984,10 @@ fn createFoo(param: i32) !Foo {
 }
       {#end_syntax_block#}
       <p>
+      The {#syntax#}errdefer{#endsyntax#} statement can optionally capture the error:
+      </p>
+      {#code|test_errdefer_capture.zig#}
+      <p>
       The neat thing about this is that you get robust error handling without
       the verbosity and cognitive overhead of trying to make sure every exit path
       is covered. The deallocation code is always directly following the allocation code.

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