[clang] [NFC][Clang][Docs] Update Pointer Authentication documentation (PR #152596)

[Oliver Hunt via cfe-commits]

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@@ -255,33 +260,160 @@ signing schema breaks down even more simply:
 It is important that the signing schema be independently derived at all signing
 and authentication sites.  Preferably, the schema should be hard-coded
 everywhere it is needed, but at the very least, it must not be derived by
-inspecting information stored along with the pointer.
+inspecting information stored along with the pointer.  See the section on
+`Attacks on pointer authentication`_ for more information.
+
 
-Language Features
+Language features
 -----------------
 
-There is currently one main pointer authentication language feature:
+There are three levels of the pointer authentication language feature:
+
+- The language implementation automatically signs and authenticates function
+  pointers (and certain data pointers) across a variety of standard situations,
+  including return addresses, function pointers, and C++ virtual functions. The
+  intent is for all pointers to code in program memory to be signed in some way
+  and for all branches to code in program text to authenticate those
+  signatures.
----------------
ojhunt wrote:

We already do sign v-table pointers, and a bunch of other data pointers (mostly objc and pending review block metadata, but also "data" pointers that are equivalent to code pointers like obj-c's SEL type). Longer term we want to implicitly sign a bunch of data pointers in things like lambdas, coroutines, etc but they don't have the same ABI constraints so have been lower priority.

https://github.com/llvm/llvm-project/pull/152596