- Add links to the GNU libc docs for `memmove`, `memcpy`, and
  `memset`, as well as internally linking to other functions in `std::ptr`
- List sources of 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

This also fixes improper text wrapping.

The rendered version does not make clear that this is a link to another
page, and it breaks the anchor link.

- Remove redundant "unsafe" from module description.
- Add a missing `Safety` heading to `read_unaligned`.
- Remove weasel words in `Undefined Behavior` description for
  `write{,_unaligned,_bytes}`.

Non-`Copy` types should not be in volatile memory.