[llvm] [LangRef] Document the difference between `<abi>` and `<pref>` (PR #147929)

[Eli Friedman via llvm-commits]

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@@ -3240,12 +3240,15 @@ as follows:
     as :ref:`Non-Integral Pointer Type <nointptrtype>` s.  The ``0``
     address space cannot be specified as non-integral.
 
+``<abi>`` provides a minimum allowed alignment for a type, and
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efriedma-quic wrote:

"pref" align is primarily used for allocas and global variables.

https://github.com/llvm/llvm-project/pull/147929