Rollup merge of rust-lang#63293 - shepmaster:align-to-doc, r=@RalfJung

Clarify align_to's requirements and obligations

Based on discussions with @RalfJung around my misunderstandings when using this.

r? @RalfJung

src/libcore/slice/mod.rs

@@ -2308,9 +2308,10 @@ impl<T> [T] {
     /// maintained.
     ///
     /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
-    /// slice of a new type, and the suffix slice. The method does a best effort to make the
-    /// middle slice the greatest length possible for a given type and input slice, but only
-    /// your algorithm's performance should depend on that, not its correctness.
+    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
+    /// length possible for a given type and input slice, but only your algorithm's performance
+    /// should depend on that, not its correctness. It is permissible for all of the input data to
+    /// be returned as the prefix or suffix slice.
     ///
     /// This method has no purpose when either input element `T` or output element `U` are
     /// zero-sized and will return the original slice without splitting anything.
@@ -2361,9 +2362,10 @@ impl<T> [T] {
     /// maintained.
     ///
     /// This method splits the slice into three distinct slices: prefix, correctly aligned middle
-    /// slice of a new type, and the suffix slice. The method does a best effort to make the
-    /// middle slice the greatest length possible for a given type and input slice, but only
-    /// your algorithm's performance should depend on that, not its correctness.
+    /// slice of a new type, and the suffix slice. The method may make the middle slice the greatest
+    /// length possible for a given type and input slice, but only your algorithm's performance
+    /// should depend on that, not its correctness. It is permissible for all of the input data to
+    /// be returned as the prefix or suffix slice.
     ///
     /// This method has no purpose when either input element `T` or output element `U` are
     /// zero-sized and will return the original slice without splitting anything.