placeholder for documentation of allocator_api library feature.

Alpha-renamed `Allocator` to `Alloc`.

src/doc/unstable-book/src/library-features/allocator_api.md

@@ -0,0 +1,15 @@
+# `allocator_api`
+
+The tracking issue for this feature is [#32838]
+
+[#32838]: https://github.com/rust-lang/rust/issues/32838
+
+------------------------
+
+Sometimes you want the memory for one collection to use a different
+allocator than the memory for another collection. In this case,
+replacing the global allocator is not a workable option. Instead,
+you need to pass in an instance of an `Alloc` to each collection
+for which you want a custom allocator.
+
+TBD