deduplicate pointer docs with include!

[lolbinarycat]

A lot of the functions in const_ptr and mut_ptr are functionally the same with nearly identical documentation.

Currently, the docs are simply duplicated across both functions, but this can cause issues, such docs getting out of sync, or even functions having identical documentation despite having different safety invariants.

I think we should use the #[doc = include!("file.md")] pattern here, similar to what is done with the c ffi types.