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Add description for Zihintntl ISA extension[1].

Link: https://drive.google.com/file/d/13_wsN8YmRfH8YWysFyTX-DjTkCnBd9hj/view

 [1]
Signed-off-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231114141256.126749-15-cleger@rivosinc.com

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>