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/libcollections/lib.rs

@@ -113,7 +113,7 @@ mod std {
 }
 
 /// An endpoint of a range of keys.
-#[unstable(feature = "collections_bound", issue = "27711")]
+#[unstable(feature = "collections_bound", issue = "27787")]
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub enum Bound<T> {
     /// An inclusive bound.

src/libcollections/string.rs

@@ -734,7 +734,8 @@ impl String {
     ///
     /// Note that this will drop any excess capacity.
     #[unstable(feature = "box_str",
-               reason = "recently added, matches RFC")]
+               reason = "recently added, matches RFC",
+               issue = "27785")]
     #[deprecated(since = "1.4.0", reason = "renamed to `into_boxed_str`")]
     pub fn into_boxed_slice(self) -> Box<str> {
         self.into_boxed_str()

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"))
 }

src/libstd/macros.rs

@@ -168,7 +168,7 @@ macro_rules! try {
 ///
 /// For more information about select, see the `std::sync::mpsc::Select` structure.
 #[macro_export]
-#[unstable(feature = "mpsc_select")]
+#[unstable(feature = "mpsc_select", issue = "27800")]
 macro_rules! select {
     (
         $($name:pat = $rx:ident.$meth:ident() => $code:expr),+