[docs] Document the --print-passes flag in opt.

Reviewed By: aeubanks, asbirlea

Differential Revision: https://reviews.llvm.org/D109663

Author: fcambus

llvm/docs/CommandGuide/opt.rst

@@ -104,6 +104,10 @@ OPTIONS
 
      opt -load=plugin.so -help
 
+.. option:: -print-passes
+
+ Print all available passes and exit.
+
 EXIT STATUS
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