add notes about alignment-altering reallocs to Allocator docs

Author: autumnontape

library/core/src/alloc/mod.rs

@@ -173,6 +173,8 @@ pub unsafe trait Allocator {
     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
     /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
     ///
+    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
+    ///
     /// [*currently allocated*]: #currently-allocated-memory
     /// [*fit*]: #memory-fitting
     ///
@@ -234,6 +236,8 @@ pub unsafe trait Allocator {
     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
     /// * `new_layout.size()` must be greater than or equal to `old_layout.size()`.
     ///
+    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
+    ///
     /// [*currently allocated*]: #currently-allocated-memory
     /// [*fit*]: #memory-fitting
     ///
@@ -296,6 +300,8 @@ pub unsafe trait Allocator {
     /// * `old_layout` must [*fit*] that block of memory (The `new_layout` argument need not fit it.).
     /// * `new_layout.size()` must be smaller than or equal to `old_layout.size()`.
     ///
+    /// Note that `new_layout.align()` need not be the same as `old_layout.align()`.
+    ///
     /// [*currently allocated*]: #currently-allocated-memory
     /// [*fit*]: #memory-fitting
     ///