add compat note about changed Vec allocation behavior

Author: the8472

RELEASES.md

@@ -78,6 +78,10 @@ Compatibility Notes
   code which expect a particular structure after `.to_string()`, rather than just arbitrary Rust code.
 - [Remove asmjs](https://github.com/rust-lang/rust/pull/117338/)
 - [Make `IMPLIED_BOUNDS_ENTAILMENT` into a hard error from a lint](https://github.com/rust-lang/rust/pull/117984/)
+- [Vec's allocation behavior was changed when collecting some iterators](https://github.com/rust-lang/rust/pull/110353)
+  Allocation behavior is currently not specified, nevertheless changes can be surprising.
+  See [`impl FromIterator for Vec`](https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#impl-FromIterator%3CT%3E-for-Vec%3CT%3E)
+  for more details.
 
 Maybe:
 - [rustc_llvm: Link to libkstat on Solaris/SPARC](https://github.com/rust-lang/rust/pull/114224/)