Add long explanation for E0547

compiler/rustc_error_codes/src/error_codes/E0547.md

@@ -0,0 +1,27 @@
+#![stable(since = "1.0.0", feature = "test")]
+#[unstable(feature = "_unstable_fn")] // invalid
+fn _unstable_fn() {}
+#[rustc_const_unstable(feature = "_unstable_const_fn")] // invalid
+fn _unstable_const_fn() {}
+```
+
+To fix the issue you need to provide the `issue` field.
+
+```
+#![feature(staged_api)]
+#![stable(since = "1.0.0", feature = "test")]
+#[unstable(feature = "_unstable_fn", issue = "none")] // ok!
+fn _unstable_fn() {}
+#[rustc_const_unstable(
+    feature = "_unstable_const_fn",
+    issue = "none"
+)] // ok!
+fn _unstable_const_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