[llvm] [llvm][docs] Expand section on non-integral pointers (PR #73242)

[Sanjoy Das via llvm-commits]

sanjoy wrote:

Conceptually looks fine to me but please wait for the other reviewers to take a look.  I've not worked in this stuff for more than five years so it's pretty paged out.

https://github.com/llvm/llvm-project/pull/73242