Rollup merge of #136259 - hkBst:patch-30, r=thomcc

Cleanup docs for Allocator

This is an attempt to remove ungrammatical constructions and clean up the prose. I've sometimes had to try hard to understand what was being stated, so it is possible that I've misunderstood the original meaning. In particular, I did not see a difference between:
 - the borrow-checker lifetime of the allocator type itself.
 - as long as at least one of the allocator instance and all of its clones has not been dropped.

library/core/src/alloc/mod.rs

@@ -49,26 +49,26 @@ impl fmt::Display for AllocError {
 /// An implementation of `Allocator` can allocate, grow, shrink, and deallocate arbitrary blocks of
 /// data described via [`Layout`][].
 ///
-/// `Allocator` is designed to be implemented on ZSTs, references, or smart pointers because having
-/// an allocator like `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
+/// `Allocator` is designed to be implemented on ZSTs, references, or smart pointers.
+/// An allocator for `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
 /// allocated memory.
 ///
-/// Unlike [`GlobalAlloc`][], zero-sized allocations are allowed in `Allocator`. If an underlying
-/// allocator does not support this (like jemalloc) or return a null pointer (such as
-/// `libc::malloc`), this must be caught by the implementation.
+/// In contrast to [`GlobalAlloc`][], `Allocator` allows zero-sized allocations. If an underlying
+/// allocator does not support this (like jemalloc) or responds by returning a null pointer
+/// (such as `libc::malloc`), this must be caught by the implementation.
 ///
 /// ### Currently allocated memory
 ///
-/// Some of the methods require that a memory block be *currently allocated* via an allocator. This
-/// means that:
+/// Some of the methods require that a memory block is *currently allocated* by an allocator.
+/// This means that:
+///  * the starting address for that memory block was previously
+///    returned by [`allocate`], [`grow`], or [`shrink`], and
+///  * the memory block has not subsequently been deallocated.
 ///
-/// * the starting address for that memory block was previously returned by [`allocate`], [`grow`], or
-///   [`shrink`], and
-///
-/// * the memory block has not been subsequently deallocated, where blocks are either deallocated
-///   directly by being passed to [`deallocate`] or were changed by being passed to [`grow`] or
-///   [`shrink`] that returns `Ok`. If `grow` or `shrink` have returned `Err`, the passed pointer
-///   remains valid.
+/// A memory block is deallocated by a call to [`deallocate`],
+/// or by a call to [`grow`] or [`shrink`] that returns `Ok`.
+/// A call to `grow` or `shrink` that returns `Err`,
+/// does not deallocate the memory block passed to it.
 ///
 /// [`allocate`]: Allocator::allocate
 /// [`grow`]: Allocator::grow
@@ -77,32 +77,28 @@ impl fmt::Display for AllocError {
 ///
 /// ### Memory fitting
 ///
-/// Some of the methods require that a layout *fit* a memory block. What it means for a layout to
-/// "fit" a memory block means (or equivalently, for a memory block to "fit" a layout) is that the
+/// Some of the methods require that a `layout` *fit* a memory block or vice versa. This means that the
 /// following conditions must hold:
-///
-/// * The block must be allocated with the same alignment as [`layout.align()`], and
-///
-/// * The provided [`layout.size()`] must fall in the range `min ..= max`, where:
-///   - `min` is the size of the layout most recently used to allocate the block, and
-///   - `max` is the latest actual size returned from [`allocate`], [`grow`], or [`shrink`].
+///  * the memory block must be *currently allocated* with alignment of [`layout.align()`], and
+///  * [`layout.size()`] must fall in the range `min ..= max`, where:
+///    - `min` is the size of the layout used to allocate the block, and
+///    - `max` is the actual size returned from [`allocate`], [`grow`], or [`shrink`].
 ///
 /// [`layout.align()`]: Layout::align
 /// [`layout.size()`]: Layout::size
 ///
 /// # Safety
 ///
-/// * Memory blocks returned from an allocator that are [*currently allocated*] must point to
-///   valid memory and retain their validity while they are [*currently allocated*] and the shorter
-///   of:
-///   - the borrow-checker lifetime of the allocator type itself.
-///   - as long as at least one of the instance and all of its clones has not been dropped.
+/// Memory blocks that are [*currently allocated*] by an allocator,
+/// must point to valid memory, and retain their validity while until either:
+///  - the memory block is deallocated, or
+///  - the allocator is dropped.
 ///
-/// * copying, cloning, or moving the allocator must not invalidate memory blocks returned from this
-///   allocator. A copied or cloned allocator must behave like the same allocator, and
+/// Copying, cloning, or moving the allocator must not invalidate memory blocks returned from it
+/// A copied or cloned allocator must behave like the original allocator.
 ///
-/// * any pointer to a memory block which is [*currently allocated*] may be passed to any other
-///   method of the allocator.
+/// A memory block which is [*currently allocated*] may be passed to
+/// any method of the allocator that accepts such an argument.
 ///
 /// [*currently allocated*]: #currently-allocated-memory
 #[unstable(feature = "allocator_api", issue = "32838")]