Rollup merge of rust-lang#95546 - autumnontape:allocator-realloc-alig…

…n-docs, r=Amanieu

add notes about alignment-altering reallocations to Allocator docs

As I said in rust-lang/wg-allocators#97, the fact that calls to `grow`, `grow_zeroed`, and `shrink` may request altered alignments is surprising and may be a pitfall for implementors of `Allocator` if it's left implicit. This pull request adds a note to the "Safety" section of each function's docs making it explicit.

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
     ///