Rollup merge of #145260 - SabrinaJewson:vec-allocator-docs, r=dtolnay

Make explicit guarantees about `Vec`’s allocator

This commit amends the documentation of `Vec::as_mut_ptr` and `Vec::into_raw_parts` to make it explicit that such calls may be paired with calls to `dealloc` with a suitable layout. This guarantee was effectively already provided by the docs of `Vec::from_raw_parts` mentioning `alloc`.

Additionally, we copy-paste and adjust the “Memory layout” section from the documentation of `std::boxed` to `std::vec`. This explains the allocator guarantees in more detail.

Author: Zalathar