[Docs] Release note for LLVM_ENABLE_SYMBOLIZER_MARKUP

Author: mysterymath

llvm/docs/ReleaseNotes.rst

@@ -181,6 +181,13 @@ Other Changes
   see https://reviews.llvm.org/D157150 and https://reviews.llvm.org/D157151 for
   details.
 
+* On Linux, FreeBSD, and NetBSD, setting the environment variable
+  ``LLVM_ENABLE_SYMBOLIZER_MARKUP`` causes tools to print stacktraces using
+  :doc:`Symbolizer Markup <SymbolizerMarkupFormat>`.
+  This works even if the tools have no embedded symbol information (i.e. are
+  fully stripped); :doc:`llvm-symbolizer <CommandGuide/llvm-symbolizer>` can
+  symbolize the markup afterwards using ``debuginfod``.
+
 External Open Source Projects Using LLVM 15
 ===========================================