io::ErrorKind: rationalise ordering in main enum

It is useful to keep some coherent structure to this ordering.  In
particular, Other and Uncategorized should be next to each other, at
the end.

Also it seems to make sense to treat UnexpectedEof and OutOfMemory
specially, since they are not like the other errors (despite
OutOfMemory also being generatable by some OS errors).

So:
 * Move Other to the end, just before Uncategorized
 * Move Unsupported to between Interrupted and UnexpectedEof
 * Add some comments documenting where to add things

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/io/error.rs

@@ -261,6 +261,33 @@ pub enum ErrorKind {
     #[stable(feature = "rust1", since = "1.0.0")]
     Interrupted,
 
+    /// This operation is unsupported on this platform.
+    ///
+    /// This means that the operation can never succeed.
+    #[stable(feature = "unsupported_error", since = "1.53.0")]
+    Unsupported,
+
+    // ErrorKinds which are primarily categorisations for OS error
+    // codes should be added above.
+
+    /// An error returned when an operation could not be completed because an
+    /// "end of file" was reached prematurely.
+    ///
+    /// 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.
+    #[stable(feature = "read_exact", since = "1.6.0")]
+    UnexpectedEof,
+
+    /// An operation could not be completed, because it failed
+    /// to allocate enough memory.
+    #[stable(feature = "out_of_memory_error", since = "1.54.0")]
+    OutOfMemory,
+
+    // "Unusual" error kinds which do not correspond simply to (sets
+    // of) OS error codes, should be added just above this comment.
+    // `Other` and `Uncategorised` should remain at the end:
+
     /// A custom error that does not fall under any other I/O error kind.
     ///
     /// This can be used to construct your own [`Error`]s that do not match any
@@ -274,26 +301,6 @@ pub enum ErrorKind {
     #[stable(feature = "rust1", since = "1.0.0")]
     Other,
 
-    /// An error returned when an operation could not be completed because an
-    /// "end of file" was reached prematurely.
-    ///
-    /// 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.
-    #[stable(feature = "read_exact", since = "1.6.0")]
-    UnexpectedEof,
-
-    /// This operation is unsupported on this platform.
-    ///
-    /// This means that the operation can never succeed.
-    #[stable(feature = "unsupported_error", since = "1.53.0")]
-    Unsupported,
-
-    /// An operation could not be completed, because it failed
-    /// to allocate enough memory.
-    #[stable(feature = "out_of_memory_error", since = "1.54.0")]
-    OutOfMemory,
-
     /// Any I/O error from the standard library that's not part of this list.
     ///
     /// Errors that are `Uncategorized` now may move to a different or a new