Auto merge of #28038 - durka:grep-unstable-issue-refs, r=alexcrichton

After submitting #28031, I ran a [script](https://gist.github.com/durka/a5243440697c780f669b) on the rest of src/ and found some anomalies. In this PR are the fixes that I thought were obvious (but I might be wrong!). The others I've submitted in issue #28037.

src/libcore/macros.rs

@@ -239,7 +239,8 @@ macro_rules! writeln {
 /// ```
 #[macro_export]
 #[unstable(feature = "core",
-           reason = "relationship with panic is unclear")]
+           reason = "relationship with panic is unclear",
+           issue = "27701")]
 macro_rules! unreachable {
     () => ({
         panic!("internal error: entered unreachable code")
@@ -301,7 +302,8 @@ macro_rules! unreachable {
 /// ```
 #[macro_export]
 #[unstable(feature = "core",
-           reason = "relationship with panic is unclear")]
+           reason = "relationship with panic is unclear",
+           issue = "27701")]
 macro_rules! unimplemented {
     () => (panic!("not yet implemented"))
 }