Add long explanation for E0543

compiler/rustc_error_codes/src/error_codes/E0543.md

@@ -0,0 +1,35 @@
+The `reason` value is missing in a stability attribute.
+
+Erroneous code example:
+
+```compile_fail,E0543
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[stable(since = "0.1.0", feature = "_deprecated_fn")]
+#[rustc_deprecated(
+    since = "1.0.0"
+)] // invalid
+fn _deprecated_fn() {}
+```
+
+To fix this issue, you need to provide the `reason` field. Example:
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+
+#[stable(since = "0.1.0", feature = "_deprecated_fn")]
+#[rustc_deprecated(
+    since = "1.0.0",
+    reason = "explanation for deprecation"
+)] // ok!
+fn _deprecated_fn() {}
+```
+
+See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix
+of the Book and the [Stability attributes][stability-attributes] section of the
+Rustc Dev Guide for more details.
+
+[how-rust-made-nightly]: https://doc.rust-lang.org/book/appendix-07-nightly-rust.html
+[stability-attributes]: https://rustc-dev-guide.rust-lang.org/stability.html