[llvm] [NFC][LLVM] Document and adopt variadic `isa` in a few places (PR #136869)

[Matt Arsenault via llvm-commits]

================
@@ -168,7 +174,8 @@ rarely have to include this file directly).
   The ``isa_and_present<>`` operator works just like the ``isa<>`` operator,
   except that it allows for a null pointer as an argument (which it then
   returns false).  This can sometimes be useful, allowing you to combine several
-  null checks into one.
+  null checks into one. Similar to ``isa<>`` operator, you can specify more than
+  one classes to check.
----------------
arsenm wrote:

```suggestion
  one class to check.
```

https://github.com/llvm/llvm-project/pull/136869