Documentation/features: refresh RISC-V arch support files

Documentation/features: refresh RISC-V arch support files

Support for these was added by the following commits:

  f2c9699f6555 ("riscv: Add STACKPROTECTOR supported")
  3c4697982982 ("riscv: Enable LOCKDEP_SUPPORT & fixup TRACE_IRQFLAGS_SUPPORT").
  ed48b297fe21 ("riscv: Enable context tracking")
  cbb3d91d3bcf ("riscv: Add kmemleak support")

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20200810095000.32092-1-tklauser@distanz.ch
Signed-off-by: Jonathan Corbet <corbet@lwn.net>