[llvm] [Docs][llvm-exegesis] Add documentation on recently added options (PR #75408)

[Clement Courbet via llvm-commits]

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@@ -425,6 +431,12 @@ OPTIONS
   features such as memory annotations but is currently restricted to X86-64
   on Linux.
 
+.. option:: --benchmark-repeat-count=<repeat-count>
+
+  This option enables specifying the number of times to repeat the benchmark
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legrosbuffle wrote:

"the benchmark" -> "measurements"

https://github.com/llvm/llvm-project/pull/75408