[llvm] [Docs][RISCV]Remove experimental from Smctr, Ssctr,Sdext and Sdtrig (PR #161058)

[Shaoce SUN via llvm-commits]

sunshaoce wrote:

Can we also give these extensions a `See note` like `Sscounterenw`? I think this is very convenient.

https://github.com/llvm/llvm-project/pull/161058