[llvm][docs][X86] Mention code model improvements in ReleaseNotes (llvm#76190)

Author: aeubanks

llvm/docs/ReleaseNotes.rst

@@ -172,6 +172,11 @@ Changes to the X86 Backend
 * Support ISA of ``AVX10.1-256`` and ``AVX10.1-512``.
 * ``-mcpu=pantherlake`` and ``-mcpu=clearwaterforest`` are now supported.
 * ``-mapxf`` is supported.
+* Marking global variables with ``code_model = "small"/"large"`` in the IR now
+  overrides the global code model to allow 32-bit relocations or require 64-bit
+  relocations to the global variable.
+* The medium code model's code generation was audited to be more similar to the
+  small code model where possible.
 
 Changes to the OCaml bindings
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