Add issue number to read_exact unstable declarations

Author: cesarb

src/libstd/io/error.rs

@@ -153,7 +153,7 @@ pub enum ErrorKind {
     /// This typically means that an operation could only succeed if it read a
     /// particular number of bytes but only a smaller number of bytes could be
     /// read.
-    #[unstable(feature = "read_exact", reason = "recently added")]
+    #[unstable(feature = "read_exact", reason = "recently added", issue = "27585")]
     UnexpectedEOF,
 
     /// Any I/O error not part of this list.

src/libstd/io/mod.rs

@@ -363,7 +363,7 @@ pub trait Read {
     /// # Ok(())
     /// # }
     /// ```
-    #[unstable(feature = "read_exact", reason = "recently added")]
+    #[unstable(feature = "read_exact", reason = "recently added", issue = "27585")]
     fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
         while !buf.is_empty() {
             match self.read(buf) {