doc: Update doc for commands "riscv expose_csrs" and "riscv expose_custom"

doc: Update doc for commands "riscv expose_csrs" and "riscv expose_custom"

These commands were extended/improved in the last drop
of RISC-V target updates. Update also the documentation
to properly describe how the commands should be used now.

Change-Id: I9e2ba6adbe1a4c032b96f5f8ff2d4791fa4c2527
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6738
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tim Newsome <tim@sifive.com>