Documentation/features: refresh RISC-V arch support files

Support for these was added by the following commits:

  f2c9699f ("riscv: Add STACKPROTECTOR supported")
  3c469798 ("riscv: Enable LOCKDEP_SUPPORT & fixup TRACE_IRQFLAGS_SUPPORT").
  ed48b297 ("riscv: Enable context tracking")
  cbb3d91d ("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>