[llvm][Docs] Add a release note about the lldb DWARF indexing speedup (#122870)

The figure includes works that's already committed. In does not include
the WIP/RFC proposal in
https://discourse.llvm.org/t/rfc-speeding-up-dwarf-indexing-again/83979.

Author: labath

llvm/docs/ReleaseNotes.md

@@ -449,6 +449,8 @@ Changes to LLDB
           _regexp-bt        -- Show backtrace of the current thread's call ...
           _regexp-display   -- Evaluate an expression at every stop (see 'h...
   ```
+* DWARF indexing speed (for binaries not using the debug_names index) increased
+  by [30-60%](https://github.com/llvm/llvm-project/pull/118657).
 
 * Minidumps generated by LLDB now support:
   * 64 bit memory (due to 64b support, Minidumps are now paged to disk while being written).