@deprecated: add langref entry

doc/langref/test_deprecated_builtin.zig

@@ -0,0 +1,22 @@
+test "deprecated code path" {
+    compute(.greedy, false, 42);
+}
+
+const Strategy = enum { greedy, expensive, fast };
+fn compute(comptime strat: Strategy, comptime foo: bool, bar: usize) void {
+    switch (strat) {
+        .greedy => {
+            // This combination turned out to be ineffective.
+            if (!foo) @deprecated(); // use fast strategy when foo is false
+            runGreedy(foo, bar);
+        },
+        .expensive => runExpensive(foo, bar),
+        .fast => runFast(foo, bar),
+    }
+}
+
+extern fn runGreedy(foo: bool, bar: usize) void;
+extern fn runExpensive(foo: bool, bar: usize) void;
+extern fn runFast(foo: bool, bar: usize) void;
+
+// test_error=deprecated