- Add links to the GNU libc docs for `memmove`, `memcpy`, and
  `memset`, as well as internally linking to other functions in `std::ptr`
- List invariants which, when violated, cause UB for all functions
- Add example to `ptr::drop_in_place` and compares it to `ptr::read`.
- Add examples which more closely mirror real world uses for the
  functions in `std::ptr`. Also, move the reimplementation of `mem::swap`
  to the examples of `ptr::read` and use a more interesting example for
  `copy_nonoverlapping`.
- Change module level description
- Define what constitutes a "valid" pointer.
- Centralize discussion of ownership of bitwise copies in `ptr::read` and
  provide an example.

This also removes the overlong link that failed tidy xD.

They closely mirror the docs for `copy_nonoverlapping`

The enumerated list of conditions is replaced by an explanation that
rust doesn't have a formal memory model. It does say that pointers
created directly from references are guaranteed to be valid, and links
to both the "Unsafe Code" section of the book and the "Undefined
Behavior" section of the reference.

Uses `x.offset(i)` must be valid for all `i` in `0..count`.

This splits "valid" into "valid for reads" and "valid for writes", and
also adds the concept of operation size to validity. Now functions which
operate on sequences state that e.g. pointer args must be "valid for reads of
size x".

Also rewrites the reads/writes section to be less reliant on `*const`,
`*mut`

…oid duplication and inconsistency

The whole keyword docs thing is pretty new in Rust's history and needs
some work before it's a shining gem. Here's hoping I can provide that.

I basically shoved in a bunch of the most important information from the
reference and the book, along with leaving links to both at the end. I
don't think keyword docs need to have complete detail, just all the
broad strokes, so if someone's confused about a usage of a keyword they
can look at the std documentation for that keyword.

It's pretty basic and could do with more details, but it's a good
starter until someone else improves it.

Turns out writing docs on keywords that are used in multiple different
places in entirely different contexts gets a little harder. I put a
footnote on `*const` syntax just to make sure you can find it if need
be, but it might need more detail.

Mostly addressing notes on ambiguous syntax and spurious newlines.

Fixes #53676.

Copy the documentation over to `slice::from_raw_parts_mut`.

I think it might be used in some other things, but I'm not fluent enough
at sifting through the rust compiler's source code to find every use of
a specific keyword.

This leaves the question of how to document the `extern` keyword, what
with how much overlap it has with `crate`, but that's used with ABI
stuff so that should be fine.