[docs] Explain address spaces a bit more in opaque pointers doc

[tschuett]

This text explicitly does not talk about address spaces. For typed and opaque pointers, they still play the same role. For more information, see
DataLayout <LangRef.html#langref-datalayout>_.

[dexonsmith]

The new paragraph or something similar LGTM, although probably @theraven should be the one to sign off given the post-commit feedback.

In D102523#2760520, @tschuett wrote:

This text explicitly does not talk about address spaces. For typed and opaque pointers, they still play the same role. For more information, see
DataLayout <LangRef.html#langref-datalayout>_.

I'm not sure; I feel like @aeubanks's current text might be a little more useful, if the purpose is to head off the sorts of questions brought up in https://reviews.llvm.org/D102292. Pointers have two axes of variability/kinds and we're taking one away; I don't think everyone has clarity on what each one was for in the first place. IMO, this should serve as a "if you're worried about lowering, that's not changing, don't worry" kind of disclaimer, carving a distinction between the two axes. With your suggested more succinct text, the reader must already fully understand address spaces vs. pointer types to get that insight.

[aeubanks]

make more explicit that opaque pointers aren't changing lowering

[aeubanks]

ping

llvm/docs/OpaquePointers.rst

 ===============
 Opaque Pointers
 ===============
 
 The Opaque Pointer Type
 =======================
 
 Traditionally, LLVM IR pointer types have contained a pointee type. For example,
 ``i32 *`` is a pointer that points to an ``i32`` somewhere in memory. However,
 due to a lack of pointee type semantics and various issues with having pointee
 types, there is a desire to remove pointee types from pointers.
 
 The opaque pointer type project aims to replace all pointer types containing
 pointee types in LLVM with an opaque pointer type. The new pointer type is
 tentatively represented textually as ``ptr``.
 
-Anything to do with pointer address spaces is unaffected.
+Address spaces are still used to distinguish between different kinds of pointers
+where the distinction is relevant for lowering (e.g. data vs function pointers
+have different sizes on some architectures). For more information, see
+`DataLayout <LangRef.html#langref-datalayout>`_.
 
 Issues with explicit pointee types
 ==================================
 
 LLVM IR pointers can be cast back and forth between pointers with different
 pointee types. The pointee type does not necessarily actually represent the
 actual underlying type in memory. In other words, the pointee type contains no
 real semantics.