[Docs][llvm-exegesis] Add documentation for --use-dummy-perf-counters

This patch adds in documentation for the --use-dummy-perf-counters
option (introduced in D146301).

Reviewed By: kpdev42

Differential Revision: https://reviews.llvm.org/D147842

llvm/docs/CommandGuide/llvm-exegesis.rst

@@ -362,6 +362,13 @@ OPTIONS
  If set,  llvm-exegesis will dump the generated code to a temporary file to
  enable code inspection. Disabled by default.
 
+.. option:: --use-dummy-perf-counters
+
+ If set, llvm-exegesis will not read any real performance counters and
+ return a dummy value instead. This can be used to ensure a snippet doesn't
+ crash when hardware performance counters are unavailable and for
+ debugging :program:`llvm-exegesis` itself.
+
 EXIT STATUS
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