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

[Liao Chunyu via llvm-commits]

ChunyuLiao wrote:

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

They have assembly instructions, and are outside the scope of the riscv profile.
https://github.com/llvm/llvm-project/pull/120936 
https://github.com/llvm/llvm-project/pull/105148

Do we need to add the spec document to RISCVUsage.rst?
https://github.com/riscv/riscv-debug-spec/releases
https://github.com/riscv/riscv-control-transfer-records/releases/tag/v1.0_rc3

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